How Much Sex?

As I work on my novel, the eternal question keeps arising: how much sex should I include?

My natural instinct is to go light on the sex scenes.  At the moment, I have one scene that at first was pretty bold, but now is toned down.  There are, however, other spots where such scenes would fit nicely.  My novel will be aimed at your average science fiction/fantasy reader, and I think too much sex would be both unnecessary and off-putting.

On the other hand, sex sells.  It also keeps things from getting dull, creating a nice tension.

As it stands, I am thinking of writing two versions, one with light romance novel scenes, and another that gets a little more graphic.  If it comes to that, I might just put both up and see which does better.  The winner would probably be the sexier version, but it would be interesting to see the market reaction.

If anyone has any thoughts on this, I'm interested in how you handled similar issues.