If I'm hanging out with friends, everyone is there as a friend. I'm not hanging out with some friends and a couple, so I don't expect the gathering to seem as such. If I'm with two mutual friends who happen to be dating, I am immediately turned into the third wheel the second they start acting more like a couple than like friends. That's how relationships end groups of friends; shit gets awkward. The people responsible never want to admit it and just brush it off with "they were just jealous and angry. I don't need losers who get butthurt just because they're single and I'm not" every time.
tl;dr: Seriously, it's irritating when you hang out with people and they can't keep their hands off each other.