Because they know theyre straight people and they know we're a minority. Bruce/Caitlyn Jenner is still a straight man attracted to women. He's been raised with an intense disgust for gay people all his life. It's not like he's undone any indoctrination. He's also a straight man so it's not like he's magically removed all his misogyny when identifying as a woman. When he's provoked enough by a gay man, he just lets all his heterosexuality out. When he's provoked enough by a woman, he'll let all his maleness out. I know rape tweets are common on twitter but one that im reminded of when i went browsing through twitter on my own is some woman commenting on the logic of transexuality and it was pinned and retweeted (not sure what the proper term is) by a Trans Identified Male with the caption: "hold on. going outside for a sec. Where are my raping shoes". It just goes to show you they KNOW theyre not who they say they are. When 2 gay people disagree or deeply hate each other, they dont use the F-slur because theyre both gay and it cancels out so it makes no sense. But i've had "bisexual/pansexual men say it to me, straight women who "identify" as gay men. It's the first word out of their mouths when they get provoked. Any group who has the "can i say the f-slur...can i say slurs targeted towards lesbians!" would immediately use them against the intended target and they fully know it's almost always never them that it's targeted towards.