A Fork said:
Reason 1: It's Boring
Have you ever watched a kissing scene where you felt nothing for the romance, or worse it was gross, like something from PrankInvasion? Now imagine sitting through even a 1 minute sex scene. Now I've played many a VN, and I could probably count the number of ones with sex scenes with two hands, maybe a foot. Yet I've only played one or two, which I actually read through and thought, "yes Sir Elton John, I can feel the love tonight." Basically, if it ain't your waifu, chances are you won't care.
I think this really depends on the story and game as to whether its boring. Sex isn't just there to arouse you and titillate you. When it is... Most people don't care whether its their Waifu or not TBH. Porn exists for a reason, and most people I know don't just watch one actress and get bored when she's not in their porn; they get aroused by the sex itself, and the attractive bodies, whether its their waifu or not.
If the player is meant to feel an emotional bond with that character personally - then sure, it can be boring. If it is supposed to show an emotional bond between two characters, whether one is the one the player is playing {Which is a separate entity from the player themselves; you are you, the character on the screen can be a stand in for you, or can be their own character} or not, then it doesn't need to engage the player themselves on a personally emotional level, and is much easier to be non-boring.
You bring up GoT, and much as people don't rave about the sex scenes, I don't think I've heard it said "OMG those sex scenes are so boring" either. They fit the world, and the characters, and tell us things about them. They're not prudishly holding back because some viewers might be embarrassed, or trying to make a kids show - they're just showing their world, and that level of genuineness tends to be appreciated by viewers [That said it can go past that sometimes, but that's another conversation].
Reason 2: Most are Unnecessary
Since VNs probably have the most sex, I'll be talking about those I've read threads on VN sites discussing whether sex was necessary in VNs, and the response is yes, but some are unnecessary. However, if you use the word "necessary", you might as well ask "Would you choose between having sex scenes or not having sex scenes?" Obviously the answer would be, I'd rather have sex scenes, because there is one I like or may like. Still, it's basically if it ain't your waifu, you won't care.
I'm picking the main reason you've got a lot of your opinion is because you are basing it primarily of Eroge. Lets be honest here, a lot of Eroge is fan service, with a main character that is just a stand in for the player, trying to arouse the player sexually and act as porn, often unnecessarily for its plot. Now, this isn't an exclusive truth, there are some truly great eroge and VNs out there, but a lot of them are just glorified porn. When you're focusing on glorified porn, yeah, a lot of the sex is going to be unnecessary. You're looking at the wrong sort of genre for that.
See what I said about GoT above for when it can be 'necessary' to have a sex scene, or at least more detrimental to leave it out. Games naturally have their own considerations to worry about too, but by and large the rule ends up pretty similar.
Reason 3: It's Too Much
Honestly kissing and hugging is more than enough to convey a sense of romance. Being sexually aroused is distracting, to say the least. If a video game is good enough to be porn to do you know what, then you want to do you know what.
For whatever reason, I found softcore scenes to be more romantic than hardcore scenes.
Again, see what I said about GoT. Some of it isn't meant to sexually arouse you, but to build the world, and kissing and hugging in a world of excess, brutality and animal instinct tends to come off as fake, and contrast with the tone more than agree with it - I.E: Its detrimental to only go half way. Again, depends on the game, but hey.
Reason 4: Humans are Sex-Negative
Besides humans, name me a animal that has sex and masturbates in private. I think the shame of sex is universal. The word "awkward" is frequently when used to describe sex scenes, perfectly describing a faux pas. Some of us are different, but generally, we don't like sex and sexuality in public situations. At least, we avoid bringing it up with randoms.
This is just your own personal bias. Imma guess you're either being brought up in a quite conservative or Christian area, or are 30+ years old. Sex is nowhere near the taboo subject it once was, and a lot of the younger generation don't have too much problem with talking sex, to be entirely honest. Now, kinkiness and fetishes are still a bit awkward for most, but being able to say you had sex, or you jerk it at night, or TBH even a lot of flirting with some BDSM vibes can be quite acceptable. It is far from a universal shame, that's just a cultural thing in certain areas and demographics [Especially in America]. "Awkward" is used when describing sex scenes, because they tend to not be natural. They tend to try and do the just "hugging and kissing", or softcore sort of thing, and it just looks awkward because we all know that's not what sex is like. See Dragon Age; Origins and its weird sex scenes that really were just awkward, not the least of which was because they were all just grinding against each other while still wearing underpants. Again, I haven't heard anyone call GoTs sex scenes "Awkward". They're just kind of there, they're part of the world and story. There's nothing shameful about that.
tl;dr
Video game sex will never mature because sex isn't mature at all. What people think is mature is basically a bunch of people implicitly agreeing X is tasteful and you nodding your head.
What people think of as mature is not being ashamed of what you are, but also not to take it too far in excess. It isn't mature to have a basic PG romantic story, and halfway through there's this outrageous R18+ full sex kink show going on. That's simply fanservice a lot of the time.
But likewise it isn't mature to take something like Game of Thrones, and insist on everyone just hugging and kissing in their clothes on screen because we're too ashamed of the oldest and most common of human expressions of intimacy.
Mature is knowing what suits the story you're telling, and the medium, and following through on that. If you're making a more tasteful narrative, then yeah, mature is going to be tasteful. If you're making a narrative about excess and exaggeration, your sex scenes are going to be anything but tasteful.
Sex is in no way immature. Its the attitudes that our cultures built towards sex in the lead up to the 20th century that is immature. Those cultural attitudes are slowly shifting, however, and sex is becoming a more and more accepted part of our shared culture, and more people are realising that it is nothing to be ashamed of, but also not something to be implicitly proud of. It simply is what it is.
