Depends, is the sex all with the same person?
Not that I'm against monogamy, mind you, but the problem here is that Sex, while unfucking believable, doesn't have THAT much variety with the same person, I mean, you have a solid year or so of awesome, but then things start getting stale, believe me, i've been in a relationship for 5 years.
Videogames, however, are CONSTANTLY DIFFERENT, Call of Duty, Dragon Age, Oblivion, BioShock, Metal Gear, all of the variety!
Plus, you can only have sex for like two hours, then you get tired. And you can't do it again right after. It can be very tiring, if done right.
So, while sex is great, it can get boring.
And while video games are great, they aren't sex.
Tough question.