i think it would be really nice for people to not look at someone and assume that they're heterosexual. i'm not saying that you should think everyone is gay - i just don't want it to be a huge shock when someone is.
maybe i'd like to walk around with my male friends and not have people automatically think we're dating - incidentally, if i was to have a girlfriend i would not want people to assume we were just friends.
don't look at people and assume they are straight and cisgender.
don't look at people and assume anything.
human beings are more ambiguous than that - don't pretend that there is a default setting we are all programmed to because humanity as a whole is so much more than that.