Any flexitarians? How do you explain your diet to others?

I will throw another angle at this and I mean no harm. It doesn't matter to me what you eat and how. (gluten free, vegan, vegetarian, etc) When you go to a catered event, unless you have specific moral beliefs, or dietary restrictions you eat what is available, unless offered a choice up front.
I am only saying this, because I work in a restaurant that does A LOT of catering, and I work with a lot of people who organize these events. There is no way to accommodate and please everyone. You include some vegetarian options and hope you have enough. You include meat options too. It's a hard balance.
I am not trying to take away from what you are trying to accomplish. I just think that to be polite to the host, you could make sure there is enough for those people with a medical or moral need. I think it adds to the flexitarianism you are promoting.

Sorry if it seems harsh, but I would feel absolutely terrible if someone came to an event I hosted and couldn't eat anything I had available.

And still. So what? If you find it so annoying to talk about your choices, you shouldn't bring them up or you should go ahead and let people know with a "and I don't want to talk about it" clause. They'll probably take the hint that you're not down for a conversation then. Or ever.

nFor about 6 months, I've been eating primarily vegetarian, but haven't entirely given up meat, and don't intend to. I eat meat maybe 2-3 meals a week or so, usually on the weekends, usually at restaurants or take-out.

It's not a type. It's just being an omnivore, albeit an omnivore who craves a trendy attention getting label.

People without dietary restrictions, voluntary or otherwise, really have no idea how much comment is provoked by not eating meat in a group setting.

I am mainly pescetarian, although I cook almost entirely vegetarian for myself at home. But I'm not so meat-avoidant that I'll pick miniscule pieces of ham out of pea soup or tiny bits of chicken out of chicken fried rice. Somehow I always end up being labelled as "vegetarian", which seems to be the conceptual catch-all for anyone who doesn't eat everything. This mainly arises prior to planned gatherings where someone will be sure to say "Oh, I've never cooked for a vegetarian before" or something along those lines -- even though I always insist that the usual meal should be served and I will be perfectly happy just eating the side dishes (normally plentiful and varied at a large meal).

I feel your pain. I try to fly under the radar as much as possible but people do like to question and draw attention to anything out of the norm.

EXACTLY! I never insist that special food be made for me, nor do I show up with 'my own' food to a dinner party (which a lot of articles suggest, but I find that to be rude)--I am perfectly happy to eat the sides provided, not to mention that having fewer options helps me keep my calorie intake down at such events! I don't pick bacon bits out of the twice-baked potato skins at a restuarant, and more often than not, if I am offered a meat-centric dish as my only option, I will just say that I am not hungry to avoid the awkward exchange that comes from a flat-out refusal.When trying to pick a restuarant with friends, everyone looks at me and says "Where can you eat?" Uh, I can eat anywhere! I usually order fish when I'm out to eat! I am not trying to inconvenience anyone around me. But still, SOMEHOW, they respond as though my choices are a value judgement against them and that I am an outlier, a special case that needs to be treated with kid gloves and then whispered about later on. I feel like people who have food allergies or medical conditions never get a second glance about their diets, but the moment you mention that you don't really eat meat for other reasons, you are a pariah.