It's simply not.
It's one man, one woman.
You'll notice, of course, that no one is forcing all-female schools to shut down, nor all-female gyms.
No one is looking for equality. They already have that through the court system and various legal documents. What they are looking for is for the world to see NO DIFFERENCE between marriage and gay hookups.
And to that I say stick it in your ***.
It isn't? Are you sure? Ooops!