If you're using the argument that utilitarianism supported gay rights before it was cool to do so, then I feel the need to point out that I skimmed that article, and Bentham says that out of necrophilia, bestiality, homosexuality and masturbation, masturbation is the most damaging to health.

The impropriety then may consist either in making use of an object

1. Of the proper species but at an improper time: for instance, after death.

2. Of an object of the proper species and sex, and at a proper time, but in an improper part.

3. Of an object of the proper species but the wrong sex. This is distinguished from the rest by the name of paederasty.

4. Of a wrong species.

5. In procuring this sensation by one's self without the help of any other sensitive object.

...

Of all irregularities of the venereal appetite, that which is the most incontestably pernicious is one which no legislator seems ever to have made an attempt to punish. I mean the sort of impurity which a person of either sex may be guilty of by themselves. This is often of the most serious consequence to the health and lasting happiness of those who are led to practise it.

You can't just cherry pick his support for gay rights to argue that he supported modern sexual norms 200 years early.

Read your own article--I already knew what was wrong with it because Scott Alexander misunderstood it on Slate Star Codex in the same way. Bentham believes that all homosexuality is pedophiliac and that therefore homosexuals will have to marry women in order to have relationships that are long-lasting and where their partner reciprocates their affection Therefore, homosexuality doesn't harm straight marriage. That does conclude that homosexuality is harmless, but not by a chain of reasoning we would approve of.

[Utilitarianism is] very good. It's more or less reliably better than anything else.

That's a sweeping claim. A number of people have made similar points, but I'll weigh in aanyway:-

Its pretty nearly the case that there is nothing to judge an ethical theory by except intuition, and utilitarianism fares badly by that measure. (One can also judge a theory by how motivating it is, how consistent it is, and so on. These considerations might even make us go against direct intuition, but there is no point in a consistentl and/or motivating system that is basically wrong).

One problem with utilitarianism is that it tries to aggregate individual values, making it unable to handle the kinds of values that are only definable at group level, such as equality, liberty and fraternity.

Since it focuses on outcomes, it is also blind to the intention or level of deliberateness behind an act. Nothing could be more out of line with everyday practice, where "I didn't mean to" is a perfectly good excuse, for all that it doesn't change any outcomes.

Furthermore, it has problems with obligation and motivation. The claim that the greatest good is the happiness of the greatest number has intuitive force to some, but regarded as an obligation it implies one must sacrifice oneself until one is no longer happier or better off than anyone else .. it is highly demanding. On the other hand, it is not clear where the obligation comes from, since the is-ought gap has not been closed. In the negative case, utilitarianism merely suggests morally worthy actions, without making them obligatory on anyone. It has only two non arbitrary points to set a level of obligation at, zero and the maximum.

Even if the bullet is bitten, and it us accepted that “maximum possible altruism is obligatory”, the usual link between obligations and punishments is broken. It would mean that almost everyone is failling their obligations but few are getting any punishment (even social disapproval).

That's without even getting on to the problem arising from mathematically aggregating preferences, such as utility monstering, repugnant conclusions, etc.

Utilitarianism does not support anything in particular in the abstract, since it always depends on the resulting utilities, which can be different in different circumstances. So it is especially unreasonable to argue for utilitarianism on the grounds that it supports various liberties such as gay rights. Rights normally express something like a deontological claim that other people should leave me alone, and such a thing can never be supported in the abstract by utilitarianism. In particular, it would not support gay rights if too many people are offended by them, which was likely true in the past.

Utilitarianism is useful in a narrow range where we have a good utility function. The problem is easiest when the different options offer the same kind of utility. For example, if every option paid out in dollars or assets with a known dollar value, then utilitarianism provides a good solution.

But when it comes to harder problems, utilitarianism runs into trouble. The solution strongly depends on the initial choice of utility function. However, we have no apparatus to reliably measure utility. You can easily use utilitarianism to extend other moral systems by trying to devise a utility function that provides the appropriate conclusions in known cases. If I do this for Confucianism, I'm being as utilitarian as someone doing it for Enlightenment teachings.

There is insufficient basis for making such a comparison. It's highly questionable that an ethical system can be "right" in the same way that a physical theory can be "right". There is an obvious standard by which to evaluate the rightness of a scientific theory: just check whether its factual claims accurately describe reality. The "system-is" must match the "reality-is" But a moral system is made out of oughts, not descriptive statements. The "system-ought" should be matching... exactly what? We don't even know, or aren't able to talk about, our "reality-oughts" without reference to either our intuitions or our moral system. If the latter, any moral system is self-referential and thus with no necessary grounding in reality, and if the former, then our foundational morality is our system of moral intuitions, and an ethical system like utilitarianism necessarily describes it or formalises it and may be superfluous. And the entire thesis of your post is that "reality-oughts" may turn out to fly in the face of our intuitions. This undermines the only basis there is for solving the is-ought problem.

The reason you expect some morally unintuitive prescriptions to prevail seems to rely on choosing the systemically-consistent way out of extreme moral dilemmas, however repugnant it may be. Now (I should mention I'm a total pleb in physics, please contradict me if this is wrong) we generally know reality to be self-consistent by necessity, and we aspire towards building self-consistent physical models of the world, at the expense of intuitions. Doing otherwise is (?) including magic as a feature in our model of the world. In the moral realm, to accept inconsistency would be to accept hypocrisy as a necessity, which is emotionally unpalatable just like physical-system inconsistency is confusing. But it is not obvious that morality is ultimately self-consistent rather than tragic. Personally I incline towards the tragedy hypothesis. Bending over backwards for self-consistency seems to be a mistake, as evidenced by repugnant conclusions of one sort or another. The fact that your moral system pits consistency values against object-level values in extreme ethical dilemmas seems to be a strike against it rather than against those object-level values.

About utilitarianism specifically: if you have your zeitgeist-detection-goggles on, it's easy to see utilitarianism as a product of its contemporary biases: influenced by a highly economical worldview. Utility can be described as moral currency in some aspects. It does even worse in introducing glitches and absurdities than its economical counterpart, because it's a totalising ethical notion -- one which aims to touch every aspect of human existence, instead of being confined to the economic realm. Utility is a quantitative approach to value that attempts to collapse qualitatively different values into one common currency of how much satisfaction can be extracted from any of them. My go-to example for this is Yudkowsky's torture vs. dust specks, which fails to distinguish between bad and evil (nuances are, apparently, for unenlightened pre-moderns), upping the amount of badness to arbitrary levels until it supposedly surpasses evil. This kind of mindset is, at its most charitable understanding, a useful framework for a policy-maker that has to direct finite resources to alleviating either a common and slight health problem (say, common colds or allergies) or a rare and deadly disease. Again, a problem that is economic in nature, that has a dollar value. Utilitarianism is also popular around here for being more amenable to computational (AI) applications than other ethical systems. Beyond that, to hail it as the ultimate moral system is excessive and unwarranted.

I like this post, with the possibly-heterodox interpretation that by 'utilitarianism' you mean 'utilitarianism as we currently conceive it'--in particular, I think we're mostly working with a Newtonian utility function that probably gives the wrong answer regarding things like wireheading (unlike the Newtonian physicists we know this isn't right, but the successor isn't quite ready)

The actual 42-ish ethical system

42-ish? (The one a city-sized computer will come up with after millions of years of calculation, which is unfortunately completely useless because the question it's the answer to was underspecified?)