Why does an interactive medium passively present anything? Sometimes cinematics introduce characters, crucial elements of the plot or new gameplay mechanics. Oftentimes cut scenes prepare you for an important encounter by deliberately exposing you to information that will be useful later in the fight. Interacting at a high level from the very beginning would be asking a lot from the player.
But what if the cut scene was the encounter? Imagine spending hours preparing for an interaction with a character, only to have the game take control when it's finally time to show your stuff. It sounds infuriating, but it's a common way to approach sexuality in an otherwise interactive medium: You can participate in the foreplay, but when the clothes come off, most games don't trust you to finish what you started.
Whether bowing to societal pressures, de-prioritizing inessential content or simply feeling that sex makes for bad gameplay, game developers have typically been reluctant to allow player involvement in virtual sex. But it doesn't have to be that way. Assuming the ratings board didn't care and there were no moral quandaries, would interactive sex in a mainstream single player video game create a "better" experience with more emotional impact?