Some people will be tempted to buy a game because of sexual content; others will decide against it because of sexual content. I suspect that on the whole, the "Will buy game for sex" and the "Will abhor game for sex" lobbies will balance out.
Personally, I think it depends on the game. Sex has its place in life as a whole, and has had its place in books and cinema for centuries, so it would be remarkably fatuous to say that it should never appear in games.
It was included in Mass Effect in a low-key, appropriate way that furthered the roleplaying, and so suited the tone of the game. For some games, sexual content is entirely inappropriate (Lego Goes Wild, for instance, would perhaps not be a good idea), while in other games even fairly explicit content could be included if handled in an appropriate way - RPGs and adventure games aimed at an adult market are the obvious options.
What annoys me, though, is the idea that violence is acceptable while sex isn't. I once heard someone criticise an episode of CSI because "during the autopsy, a nipple was gratuitously shown for several seconds." It's called 'anatomy', mate, get used to it.
