I'm only speculating, but Shelter looks like a place for soldiers and excavators to come and go so the population thing isn't really an issue. Maybe there are women, they just aren't really mentioned because it's a turnoff for some readers even though there are very few sex scenes.
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Forgive the soapbox here... Yeah, there's a subset of people in the FVN community who seem to think that having a single female character in a gay story, even in completely nonsexual scenarios, is some sort of cardinal sin. I'm not exactly going to lose sleep over the lack of inclusion; a gay FVN is a very niche genre to begin with. It's not like they're meant to appeal to a general audience. But it's still kinda disappointing that some people in this community feel that way. Then again, sometimes works that do feature female characters (especially when there's literally only one woman) make their personalities feel half-baked and don't give them anything important to do in the plot. I guess if you know you don't actually have anything for them to do, it's better to leave them out entirely? I don't know.
I think your later conclusion fits best. There isn't typically a role for women to play in these kinds of FVN. If the sole focus is world-building immersion, then yes. However, I believe this creator is trying to flesh out characters they've already come up with and refining the dynamics between them. Trying to answer "why aren't there any women" in terms of world-building is a huge distraction for the writer's interest.
I can't really say I've read many FVNs with good female characters either. They're usually token characters that make rare appearances like Maria from ExcA or Cynthia from TSR. These characters are a nuisance to the story because they often kill the pacing and they don't offer much to the story so it's difficult to justify them being there. I still like them, they have interesting personalities, but they could do more than just make an appearance, nag you, then disappear for awhile as the main story continues. They become bland and predictable, and it makes the reader hate them in a way.
I'd say the better female character would be Virginia from Adastra. She doesn't appear often, but when she does, it's typically about her motives, her alliances, and her position as a woman in power. Personally, I don't really love her as a character either because she comes off as nagging you or her brother, but she does have some bittersweet moments.
Most creators who make FVNs have the goal to curate a roster of characters to romantically or sexually appeal to as many people as possible to gather support for their game. As these creators are usually single in person, and often only experienced in some coding and drawing. I'd say leave it to the people that want to make good, well-flushed out female characters that fit into the environment they've curated. While it certainly doesn't do it justice, it's best for the target audience that probably both want a men-love-men furry romance AND an interesting storyline that's well-immersed. Therefore, male characters take higher priority to accomplish these two tasks.