I think men are more easily turned on since, as has been said, they are affected more by visual stimuli. And it's said they think more about sex than women, possibly as a result.

And they see sex more as a conquest, perhaps. Bragging about it as teenagers...

In general, women see sex as more part of a relationship thing--and are in general more affected emotionally by a sexual experience. They are seen as "giving" to a man rather than taking.

And women can get pregnant as a result, which makes a difference.