I'm fine as long as it doesn't break up the flow of the story, or gameplay for that matter. For instance, if I'm playing an action RPG and I hook up with someone, and we start to have this long drawn out hardcore sex scene, while I might find myself aroused, it also just breaks things up. Like, going into great detail really only makes sense if things are building up to that sex scene. If I meet some bloke at the inn that I'm just wanting a roll in the hay with for the night, taking five minutes to go and show everything he's doing with me just feels unwarranted, but if I've been romancing this person all game, trying to win their heart, and we've even shown quite a bit of affection towards each other, and then we finally have our night together, showing everything can be a rather tasteful thing. A great analogy for this honestly is just with sex in general. Having a big sex scene may as well be the orgasm, while building up to it properly with well done background that makes it obvious that these two people have this great spark for each other makes it all the more satisfying and enjoyable. Having an orgasm with no build up is just kind of boring, and so for someone that I'm just having a quick throw with, a fade to black is perfectly acceptable and much more appropriate for the situation.
Now, to go with a different example, like just a hentai game, the entire point of it is the sex scenes, so everything is built around sex, so going into great detail just makes sense. But this is a completely different kind of game, so of course it should handle things in a completely different way. Each game and each story should handle sex in the way that is most appropriate. In a game like Dragon Age, eventually having sex with you partner and even showing a bit of the action seems appropriate because it's an entire system built into the game. Sex is a mechanic of the game, so for it to be done well, it just needs to be a well thought out mechanic. Sex in a Mario game would make zero sense, as there is nothing to even do with romance in the game mechanics. You could argue that's why Mario is doing what he's doing for Peach, but ultimately that's secondary to him just wanting to save the kingdom. It's completely in the background and their relationship is almost entirely built up around player inference, and so to throw it in a game would be completely uncouth.
My whole point is that in some games I'm fine, and even could want sex to be in my games. Whether others see it as mature or not is completely moot, it's something I enjoy in certain video games and would miss it if it were taken out of the ones that already do have it. Just as long as it doesn't upset story or gameplay flow or break with the tone of the rest of the game, it's perfectly acceptable to me, and if they were completely optional this would also be good as well.
I think games should work better on having more options, and more realistic ones at that. For instance, in a lot of Bioware games if you turn down someone's romantic advances, you're suddenly an asshole, which is bullshit. Maybe you really care for them, just not in that way. Or hell, maybe you do care for them in that way, but just don't want to have sex. It's unfair to act like you're some sort of monster for liking someone but not wanting to fuck them. On the other end of the spectrum, and looking at Fallout 4 in this one, don't be lazy about the romancing mechanics. You can look at the fact that you can marry each and every follower you come across as a progressive step towards inclusion with poly relationships, but the fact that all of them are cool with it just isn't realistic, and just comes across as them being too lazy to slap you on the wrist for running around having sex with everything. That's not how real relationships work unless all parties have agreed upon this setup, and it's very unlikely that every person that you meet is going to be okay with this. It might be the future, but acting like one on one relationships would somehow disappear, especially when everything is based on the American 1950 mindset, just doesn't fit in with the world building already setup in the game. If you want to make a game where this is the case, you can, but making everyone act the same in a relationship just feels lazy and pulls me out of any sort of immersion I might have in the game world.
I think I've ranted too much already, but my major point is still the same. I like dating sims, and I'll be more than happy to see them more prevalent in more RPGs with better fleshed out mechanics, and if I get sex thrown in there too, well that's just fine and dandy for me. Just keep it optional and keep it at least somewhat realistic.