There is way more sex in media than there is romance, romance was the wrong term to use for this instance. As for the rest of it, the history of non-hetero individuals and heterosexual individuals in the USA is different. For a long time (and to a lesser extent still) people had to hide who they were and pretend to be like everyone else, which is a really stressful way to live (teenage transgenders, who have the worst of it, have the highest suicide rate of anyone associated with the LGBT community which as a whole has higher rates than normal, it's pretty staggering if you look into it) and these parades were a way to let them know that they were not alone, and to give them a sense of community or release that burden that society had placed on them.
People do not want to be defined solely by their sexual orientation, but the subject matter in the case of these parades has everything to do with sexual orientation and gender identity so of course you are going to see a lot of things associated with that if you are an observer, this isn't everyday behavior you are seeing. Also let's be honest, if you get a lot of people together in a charged atmosphere no matter what the occasion, some pretty crazy stuff is probably going to happen.
