One thing I've always thought is that there isn't enough sex in games, simply because it restricts how seriously you can take the in-story romances. This isn't about sex, it's about love and how it contributes to the plot. I can only think of four games with serious in-game romances which actually culminated in a love scene- Xenogears, Persona 3 FES, Persona 4 and Prince of Persia: Sands of Time (I'm not sure what happened at the end of Warrior Within and in any case, the Prince and Kaileena never had a "romance" anyway). The Xenogears case is an especially good one- Xenogears is a particularly mature game, despite the 2D anime sprite-ish graphics, and the love scene (completely non-explicit, there's an anime cutscene where Fei and Elly kiss then we cut back to ingame graphics with Fei getting dressed while Elly is lying naked in bed) is the culmination of a love story which has been central to the plot for most of the game. Persona 3 was actually kind of mind-blowing for letting your high-school aged character sleep with up to 5 different girls (6 in the FES edition). Sure it was really only implied in most cases (Elizabeth's, on the other hand, was rather explicit) with a fade to black in the girl's room while you're told you "shared a tender moment" with the girl, but it was pretty obvious what they meant. However in all cases these scenes came at the end of their social link, which means you've spent a lot of time getting to know and building up a relationship with these girls- you're not just boning them because they're hot and in fact, THEY come on to YOU in their love scenes. Of course, this does mean that if you want 100% completion you basically have to cheat on them, which is why Persona 4 allowed you to opt to keep the relationship platonic for any of the girls you could romance (while still allowing you to sleep around if you chose).
The majority of other romances, especially in JRPGs, have all the maturity of a kids cartoon- the most erotic thing you'll see the characters do is kiss, if they're lucky; FFX for example had The Spring scene, although this isn't a game I think would have been improved by a love scene as Tidus and Yuna's romance was fairly juvenile, like a young crush and was pretty much fine as it was. I see this as a carry-over from anime storytelling where the culmination of a passionate romance is often no more than seeing the characters embrace for the first time. My thoughts are just that sex is a powerful and dramatic thing which should far more utilized in love stories, but guardedly. When you can bone practically any female that crosses your path (see: The Witcher) there's absolutely no emotional attachment formed there. To be honest, it's best left to compulsory or linear stories as making it optional generally reduces the amount of depth that's allowed to be assigned to the relationship before it becomes intrusive on the storyline: Persona 3 was able to get away with this because your character was just trying to carry on a normal school life by day while fighting monsters by night, but in Xenogears the relationship between Fei and Elly had gone back hundreds of years through reincarnation and had impacted massively on the back story, as well as the antagonists of the game.