Because when a guy cheats on a woman, she thinks, "I love that fool. He probably was seduced by her! Or he just had a moment of weakness! It won't happen again, I know!"
But when a woman cheats, guys are thinking, "Did she really just let another dude plant his flag where I already conquered? Woman with no morals! Probably went out there seducing all kinds of dudes! Who knows how many!"
In all, I think it goes to the ole "men will be men, boys will be boys". In that, it's expected for men to look for sex, even when in a relationship, so it can be forgiven when he transgresses in such a manner. But for a woman to do such a thing simply makes her immoral and out of control, so it just can't be forgiven. In society's eyes, that is.
*sigh* Men do not view sex as an emotional attachment, females usually do. By definition, you're more involved