The reason heterosexuals flaunt it because it's been around for, oh, i don't know, years. As in since the beginning of time, since man realized they need the opposite sex to live on and create life. Now there's nothing wrong with homosexuals going on flaunting their love in public, I've decided to accept that, but I will avoid any contact with them.
Other than the Greeks making up stories of some of the Greek mythology Gods having gay lovers I say that about covers where homosexuality began from then to now.
Actually, those Greeks you are referring to used to have gay sex themselves. Also, homosexuality appears in several species other than human. Pretty sure it's been around since heterosexuality has.
The good part is that if someone tries this argument on me in the school hallway, I know I can point in any random direction and find at least one heterosexual couple making out in a four-foot radius.