Why do some women think it's ok to go after someone else's man?

I understand that they probably wouldn't do it unless they had some form of encouragement and then are ones who are flat out stalkers but thats probably rare. Maybe its the thrill of something supposed to be forbidden? Still, when men can be so hurtful why cant women stand behind each other?
I use to work with this one married man who would always ask me to go on a date with him. In reply I would insist his wife comes too or if him and his wife had a sitter. This usually shut him up. I didnt know his wife but I had her back.