Because everyone know that women who sleep with more than one man at a time are immoral whores who probably used their womanly wiles to lure the man away from being faithful in marriage. Duh. Everyone knows that.
((Please note strong sarcasm))
Actually, you hear about that kind of thing from both sides, it's just men tend to be more vocal about it. Women who cheat on their husbands tend to do the same thing to their boytoys. The married person tends to use speacial reasoning for themselves ('oh, but I'd be only with you if I wasn't already married.' or 'It's different what I'm doing because my spouse isn't making me happy anymore, but I thought you were still happy with me?") and alot of times the married person even convinces themselves that they will leave their spouse for this other person, knowing deep down in the pit of their brain that they don't really intend to at all.
The guy I'm with (who I've been friends with since sophmore highschool year, if you wonder how I know all this) was another woman's 'other man' when he was 18 (she is a married woman who still aparently goes after younger guys all the time, her main tactic being to take her band off and waiting until her and the guy have that strong just-fell-in-love-so-everything-is-wonde... bond before dropping the bomb: 'Oh, by the way..')..she kept telling him she was about to leave her husband, yet he still saw them together, and they seemed pretty happy he said dispite her swearing she was miserable. She even got POed at him when a girl gave him her phone number at his work.
It didn't even end for a while after he finally ditched her. She stilled texted him asking if he missed her and crap like that. When she saw us a while later in a restaurant, she sent him a message telling him how much it 'hurt' her to see him with another woman 'so soon' (this had been nearly half a year since) and got all melodramatic about how wrong he was for moving on, though we found out later that she'd already moved on from him to a few other 'other guys', probably before they had even broke up, and yes, she was still married to her husband...
It's just the self-centered hypocracy of human nature I guess..