I've given this some thought, and I'm pretty sure that heterosexual sex and homosexual sex are not "total opposites." In fact, it's hard to think of something more similar to heterosexual sex than homosexual sex.
Two or more people: Check
Use of genitals: Check
Usually, but not necessarily, associated with intimacy: Check
Some level of body exposure: Check
If a dude is involved, there's some aspect of insertion into an orifice: Check
You can buy it: Check
The foreplay is pretty much the same.
The endgoal of the immediate action is the same.
I don't think these are total opposites.
I mean, if we're talking about a total opposite to sex between a man and a woman shouldn't it be like a dude watching football while a woman spends his money? That's completely different than sex, and might even lead to less sex since there will be a fight. Or more sex to make up from the fight, depending on the particulars of your relationship.
Or, in my case, reading your crackpot theories about the interaction between biology and marriage is the "total opposite" of sex because that's the biggest anti-****** I can imagine.