This all boils down to economics, IMO. Developers have to choose between plot and "plot
". If you want focus on the former, you play bishōjo eroge games. If you want focus on the latter, you play porn games.
If you try to balance development between the two with many characters involved, the most cost-effective scenario is a fuck-em-all. And that's just not really plausible for a believable, fleshed out "nice guy" MC in a believable world. Much easier to just make him a "not so nice guy".
See, I'm torn on this one because it all depends what you mean by "fleshed out" or "nice guy."
If your solution to the problem is to just keep throwing more lines of dialogue at it, then yes, it's more or less doomed to failure. Economically
and artistically. But a
good writer makes every single sentence pull double or triple duty. Any sentence... no, any
word that doesn't establish or reinforce character, set up or pay off an arc, or advance the overall plot is cut or combined.
When this philosophy is taken to the extreme, even the main action of a work... the car chases in a spy movie, the swordfights in a swashbuckler, or, yes, the sex in a hentai, can be leveraged to tell us something about the characters.
That's for a more or less linear Visual Novel. The more gameplay you layer on top of that, the more extra work it becomes. (Consider the case of the guy who set out to make a CYOA for his first game dev project. 10 choices, each with 2 options, no recombining paths after a choice is made, only a page of original text as the content exposed after each choice. 1024 endings. It nearly killed him.)
But if you treat MC Choices as alternate routes, rather than branches in the narrative, suddenly the "extra work" becomes less of a waste and more part of the appeal.
So. Is it possible to make a game with lots of sex and also lots of character development? I think the answer is yes. Most parodies of established IPs (at least, IMHO, the best ones,) already employ these tricks to make the character on the screen really
feel the same as the character on the show. If you can imagine a Steven Universe Future parody (taking place, lets say, roughly two years in the future,) where the only difference in the characters' personalities is that they want to have sex with each other, then congratulations! You are already imagining a Nice Guy having loads of gratuitous sex!
Depending on your writing process, it might not even require extra time or resources. It certainly shouldn't require extra sentences. What it
does require, unfortunately, is some damned fine writing chops, and the will to expend the mental energy to use them.
Whether or not the market would reward such effort is, of course, debatable. But I'd argue that the reason people are so into Overwatch characters is because they're so well-executed as
characters.