Yeah, I'm sure there are no-SO AUs where there are no women. There are plenty of stories like that in FPS, too, where the female characters just mysteriously disappear.

And yes, you've really hit the nail on the head there with the so-called open relationships. It would be one thing if the SO was a fully-realised character and it was shown that yes, this guy really does love his wife/girlfriend just as much as the dude he's waxing rhapsodical over, but you can't have the sort of OTP that fandom loves and have them be married to someone else. It reads like cheating, even if it's supposed to be an open relationship.