"When you see married couples do you go straight to seeing them having sex in your head?"
Because sex is all there is to LGBT relationships. It's how they identify themselves. I saw a woman in the supermarket with a T shirt that said in a rebus (look it up) lesbian. I thought of all the other more pleasant and less political ways she could define herself than by who she has sex with. She could have said "nurse, teacher, librarian, daughter, sister, atheist, netball player" all sorts of thing! Instead she chose her sexual preference!