What does the "Man Law" Say?

Question…for guys, is there some kind of “man law” for dating a friend’s ex? I know with girls there is an understanding that you typically steer clear of any guy that your friend has had a relationship with. It just goes without saying that those guys are off limits, especially if they had a significant relationship with them. But is the same thing true for guys. If your friend dated a girl for a while and it didn’t work out, would she be fair game? Is it a situational thing? What if your friend had a kid with this girl, would that make her off limits? I ask b/c I’ve seen a few situations where guys have pursued or at least entertained dating the ex girlfriend of one of their close friends. It makes me wonder if the dynamics of male relationships are just exponentially different. Now understand I’m not talking about a jump off or a one night stand. I’m referring to a substantial relationship that for whatever reason didn’t work out. Does that woman become fair game?