It’s exactly because he is a loser and a nobody that he is liked by many. Everybody likes an underdog story, and Stan is just that. For some of us he is more relateable than big dick charmer Jeremy and evil rich guy Seymour.
See, despite my own lack of attraction to Stan I can totally understand how he might appeal to some players, especially male players (correct me if I am wrong for assuming this applies to you), I imagine he serves as an option to self-insert for a portion of the userbase too, not everyone plays the same way.
My only issue with Stan, in regards to him as a concrete sex/romance option for Lena, is that it feels like any development with him is either at snail's pace or full throttle:
If Lena's naughty with him that one time in the beginning you'll catch him wanking the trunk to her images later and if you don't go that far nothing further really develops till the current point in the story.
What I don't understand about some of the people that like Stan (not pointing at you specifically
lifehacks) is the section that wants him to rape Louise and blackmail Lena. Given the audience on a porn forum I won't judge people for their fantasies but surely there are more options for Stan than that of the complete creep?
I've said previously that I wouldn't give Stan the time of day unless he improves his situation himself, by that I mean that I'd want something to happen to prove that Stan could be more than "a loner who never leaves his room" to Lena.
This doesn't necessarily have to result in a romance, it could be purely sexual too.
For example Stan could stand up for Lena in some way or show his ability to function normally, it could be a specific moment that shows his character when it comes down to it, or it could develop as the early chapters indicated; with continuous pleasant conversation options for him and Lena to interact which, might finally give him the bravery to ask her on a date (or something) of his own accord. I could imagine playing such a route, where Stan gets to the point where he braves whatever social anxiety he has, to ask out the girl that doesn't treat him like crap.
While I won't ever be physically attracted to Stan by his current design I could totally play his route, sexual scenes and all, if it ran along that kind of theme.