Women have their own pro circuit and really, when watching any sport, the most exciting events to watch are when people are evenly matched. I wouldn't want to watch a mixed event. Most people wouldn't because they like to watch fierce competition. And when people don't watch, the "pro" in pro cycling no longer exists because the riders are just two-legged advertisements. The advertising companies want the most bang for their buck. Just look at Trek sales (and the Madone in particular) because of Lance.

But really, as someone mentioned above, men are just much stronger riders. It doesn't diminish what women are capable of, it's just different.