It matters more in yaoi because when a clingy girl wedges herself between a guy and a girl, she has hope, but when she wedges herself between two gay men, she lacks the necessary equipment to make either happy, which means she has no hope. So, she has not only wasted her time and energy, she has also caused an unnecessary rift between two lovers for no reason. That makes her far more selfish than the girl who has hope and thus it is far more reprehensible.This also applies to real life as well.
As a side note: I don't know if you know many gay men, but I do. And if you have never seen a jealous gay man, especially one who is jealous over a straight chick clinging to his lover, I'll tell you now. It ain't pretty and I have never seen it end well... for the straight chick. That's just my experience.
Why can't they just ONCE in a Yaoi see a woman and a man and NOT immediately assume they're a couple... But then again I understand because it's Yaoi and Yaoi ain't good without some delicious jealousy but still. Man I hate those over-clingy girls in Yaoi...