Probably not, no.

Gay marriage was an option as far back FO2 from 1998 (could have been in F01 tool, I just remember the specific location and quest cluster/chain in F02).

No — to both games.

While I appreciate that a thing like a "Pride City" exists, it doesn't make sense in a post-apocalyptic world, where the population has been decimated. Acceptance of homosexuality isn't the problem here. If anything, I think it would be more accepted for people of the same sex to enjoy one another's company. However, completely banning heterosexuality (lubrication or death — from the link) would be going against humanity and its future. I wonder if there's some nefarious ulterior motive behind it, or if it's just a mirror of homophobia in the real world. Either way, no, I don't think it makes sense in either world. Though I haven't played the Metro games, I assume it's basically Russian Fallout, post-nuclear RPG stuff, like a casual STALKER.