I certainly agree that we shouldn't use games as a way of teaching children about appropriate sexual conduct, and I agree that it can never substitute for being in a real romantic relationship. A game's goal is to present an engaging and entertaining simulation of something. With human relationships, games have to be kind of abstract.
For example, the Sims (I use it as an example because I am familiar with it, more so then with Dating Sims) represents human interaction using points. Different activities earn you or loose you points with different people. Having a friendly conversation earns you points. Pushing someone looses you points. If you get enough points, new forms of interaction become possible and you can move from talking talking, to hugging, to kissing, to sex. Now, this isn't an accurate portrayal of human feelings. I don't think "that girl is up to 75 points with me right now, I think I wouldn't object if she tried to kiss me." However, it's not terrible as a game representing something that is actually quite complex. The same thing is done with all other aspects of games. For example, a lot of stuff can happen on a battle field, but a game which tries to portray a battle has to narrow it down to variables: hit points, attack, defense, speed, bonuses, etc. No matter how many variables you add, it won't be completely accurate but it can get close.
The other option is, of course, that sex can be part of the narrative instead of part of the game. For example, many RPGs have romantic subplots, and sometimes even sexual ones. The player doesn't have to do anything to initiate these subplots, they just happen, and how accurately they portray human sexual relationships is entirely contingent on the writers of the game.