I had the same (brief) experience with an ex-girlfriend. She just didn't care about sex that much. She enjoyed it for a while, but couldn't believe anyone would want to have sex for more than (say) 20-30 minutes. We would cuddle a lot though, and she loved that.
Quite a contrast with another ex-girlfriend; we'd be at it for hours... But that relationship was a bit *too* sexual.
So yes, some people simply don't care for the whole intercourse thing (including foreplay and such). I don't know if it's psychological or fysiological; I suppose that depends on the individual.