I agree with you, and also, the rationalist community seems unusually willing to buy things from friends with money. Bountied rationality exists. I know more than one couple that will pay each other to do things one wants but the other finds unpleasant. The "happy price" meme encourages people to ask for much more money and avoid a social obligation to give a friend price (which is kind of a gift). 

the median self-identified earner-to-give only donates about 5% of their income (IIRC, I can't find the data now)

isn't that brought down by students?

reason 16: you hate board games

#sixseasonsandamovie

The only knowledge "You might enjoy playing it" is displaying is that you know they like board games, which is not enough to be meaningful. In order to qualify as knowing the person it needs to be much more specific. And indeed, I value "you should read this book in particular because of x, y, z" much more than the median book someone might buy me. 

To rephrase more neutrally: there's a trade off between optionality and the opportunities that can only be unlocked through long commitment (analogous to rabbit hunters vs stag hunters, but over a prolonged period). Assume there's a Pareto frontier and one's position on it is morally neutral: high-commiters/stag-hunters are still better off if they pair with each other than with high-optionality-types/rabbit-hunters (although the reverse is much less true). It sucks wanting to stag hunt when everyone around you wants rabbits. Monogamy can be useful as a costly signal of "I want to stag hunt" even for someone who would be fine being poly with another stag-hunter. 

I can think of at least one friend who self-describes as not feeling jealousy and being more naturally poly, but chose monogamy for basically this reason. 

REASONS BESIDES JEALOUSY TO NOT BE POLYAMOROUS

Recently Amanda Bethlehem published comparing monogamous jealousy to kidney disease. Eneasz Brodski doubled down on this. I disagree with a lot of their implications, but today I'm going to focus on the implicit claim that jealousy is the only reason to be monogamous. Here is a list of other reasons you might choose monogamy:

1. Your sex and romance needs are satisfied by one person, and extra isn't that valuable to you (aka "polysaturated at 1", or in the case of one guy I know, at 0)
2. You + your partner are capable of allowing cuddling with friends and friendship with exes without needing to make everything allowed.
3. You are busy.
4. You hate coordination labor/value simplicity. I have been with my partner for over 4 years and we have never shared calendars. He knows when my D&D night is and I know when his house meeting is, and otherwise we work it out over text. The only reason sharing calendars would occur to me is exposure to poly.
5. You hate the idea of people you didn't choose having a significant effect on your life.
6. Or maybe you're conceptually fine with that, but you/your partner are bad at that kind of coordination
7. You are immunocompromised. STDs get all the attention, but poly also facilities transmission of simple respiratory illnesses. Your primary's close secondary's tertiary relationship is probably not going to give up concerts even if colds are devastating to you.
8. You take breakups really hard. Maybe to the point that dating isn't worth it for you, or maybe it's worth it for you but only because your partner supports you through it and they're not up for 4 months of avoidable depression per year.
9. You landed one great primary relationship but have shit taste in general. If you date, it will be a shitshow.
10. You really want the benefits of a strong primary/mono relationship, and are incapable of keeping secondary relationships small. The easiest way to protect your relationship is to not start another.
11. Your job requires a clean public image and you value that more than extra relationships
12. You lack the communication skill to do it well, and developing it is not your top priority.
13. Having your partner as your only source of sex/romance makes you try harder in ways you endorse.
14. Maybe you'll be poly someday but right now you want to establish security with each other.
15. You could get over your jealousy but other emotional growth opportunities are higher priority. 

It's a tenet of LessWrong that factual content and emotional valence are separate axes. Or more plainly, disagreeing on a matter of fact never makes you an asshole, but delivery can. 

Is it possible to take actions that cause people to dismiss something, without being sneering?

Could you define sneering, as you use it? It sounds to me like you mean something like "dismissing in entirety", which is not my definition.