I would have forfeited too, and it has nothing to do with faith or fear of losing. Anybody who's ever played a co-ed sport, even casually, whether it's basketball, volleyball, hockey, etc., knows that you just don't go 100% against girls. It's an unwritten rule. Of course usually, they don't want you to take it easy on them, but it doesn't matter. If you don't feel comfortable and can't compete at the highest level you're capable of, then it's better not to compete at all.
Even if the girl is really good and you suck, you're still a physically superior and fundamentally different male, and even if she doesn't get hurt, the competition is on uneven ground from the outset.
Is it sexist? Maybe so, but I've been known to be a little bit sexist when it's called for. It just doesn't make sense for guys and girls to compete against each other in sports unless it's just for fun/recreation.
-J