This is a curate's egg movie.
The original title, A Bet's a Bet, is far more relative. The Opposite Sex means absolutely nothing and makes no sense. But the implementation of the conceit, however you look at it, is dire! It's an interesting idea, poorly executed.
In parts, it's preachy, manipulative, and not particularly funny where it's supposed to be (apart from the intro).
But Mena Suvari is absolutely gorgeous (if that's not a too non-PC thing to say) and is one of the best things about this movie! (But in the movie she's obviously far, far, far too good for the main guy.)
Next best thing is Debra Jo Rupp who is vastly under-used, under-represented and plays a superb, far too short part. Why couldn't the director see this and write more for her?
So disappointing overall, mainly due to a so-so script. Not quite a feel-good movie, not quite a rom-com and not quite a, er, well, anything else, really...