What really goes on, I think for most people and certainly myself, is compartmentalization. I understand certain things to be ethical and others to be unethical, and when it comes time to make a decision (eating meat, for instance) that question is entirely neglected, or skimmed over.
Now, clearly animal suffering is something I don't really care about. But that doesn't mean I have any argument or foundation for believing that it is legitimately unimportant. I think this is much truer for an issue I care more about (but not enough to act fully ethically), poverty and altruism. I know that people across the world are impovershed and could benefit from my altruism more than I will benefit from something frivolous and overpriced I might buy instead. But I may still buy the frivolous thing at times.
And all but the most committed people will behave this way most of the time; they will not even earnestly try to behave ethically, but instead behave conveniently.
One possibility is that you forget that utilitarianism is correct every time you have the opportunity to buy or eat meat, but this seems unlikely. Another possibility is that you forget that meat-eating is bad from a utilitarian perspective when you have an opportunity to eat meat, but this is also unlikely.
Yes, these are both unlikely, but replace "forget" with "habitually conspire with myself to forget/ignore/brush off".
Think of it this way: Whether someone sticks to a diet (for heatlh, let's say, and not vegetarianism) or not is partly a matter of belief in the importance of the diet, but it is also partly a matter of habit, convenience, impulse and opportunity. The same is true for when we follow our ethical beliefs.
Compartmentalization does make it sound that you forget that eating meat is unethical when it's decision time.
Now, clearly animal suffering is something I don't really care about. But that doesn't mean I have any argument or foundation for believing that it is legitimately unimportant.
Do you need an argument for believing it's legitimately unimportant? Why not just say that it's an arbitrary taste? The same goes for altruism - other people may benefit more from your money than you do, but, you don't care nearly as much about them as you care about your...
Neal Stephenson's The Diamond Age takes place several decades in the future and this conversation is looking back on the present day:
I'm not sure if I agree with this characterization of the current political climate; in any case, that's not the point I'm interested in. I'm also not interested in moral relativism.
But the passage does point out a flaw which I recognize in myself: a preference for consistency over actually doing the right thing. I place a lot of stock--as I think many here do--on self-consistency. After all, clearly any moral code which is inconsistent is wrong. But dismissing a moral code for inconsistency or a person for hypocrisy is lazy. Morality is hard. It's easy to get a warm glow from the nice self-consistency of your own principles and mistake this for actually being right.
Placing too much emphasis on consistency led me to at least one embarrassing failure. I decided that no one who ate meat could be taken seriously when discussing animal rights: killing animals because they taste good seems completely inconsistent with placing any value on their lives. Furthermore, I myself ignored the whole concept of animal rights because I eat meat, so that it would be inconsistent for me to assign animals any rights. Consistency between my moral principles and my actions--not being a hypocrite--was more important to me than actually figuring out what the correct moral principles were.
To generalize: holding high moral ideals is going to produce cognitive dissonance when you are not able to live up to those ideals. It is always tempting--for me at least--to resolve this dissonance by backing down from those high ideals. An alternative we might try is to be more comfortable with hypocrisy.
Related: Self-deception: Hypocrisy or Akrasia?