There are two basic arguments for shutting the fuck up about cishets at Pride.
First: What if a trans kid asks their parents to show their support by attending Pride with them? What if a lesbian can only attend pride if she gets a ride from someone and the only person willing and able to drive her is her straight brother? What if a bi disabled person can't attend a large outdoor event without hands-on assistance from their straight partner? What if someone just wants to bring their fucking friends? What if, contrary to popular tumblr discourse, most queers don't inhabit perfectly pure social bubbles populated only by other queers?
Second and perhaps more important: If you think you can tell that someone is CIS, let alone HET, by LOOKING at them, you are a cop and an idiot.
Even if a legitimately cis and straight person is attending Pride all by themselves and not coming with their queer friends, but being legitimately supportive, they're a great reminder to the people in the community who are afraid that society in general hates us. That fear keeps people closeted and it literally kills people. The more people stand with us in good faith, the easier it is for people to feel safe to come out.
