The line in the OP that gets me is "they are not married, but have a kid together". I guess if they wanted to ensure fidelity in their relationship, they should have said the vows that I said to me wife 25 years ago and have been faithful to all that time.
Marriage is the contract that spells out the fidelity clause, co-habitating and "having a kid together" doesn't mean anything in the eyes of the law concerning fidelity.

If they were married and commited this act, THEN he would be a dirtbag.

That is rather harsh... I prefer the word "slut" :p

You can pretty much count on people to do what they do.

All labeling aside, we know what we can count on these people to do in these circumstances, and what they might probably do in future similar circumstances.

In general, someone who would cheat with you would probably also cheat on you. Dirtbag isn't very descriptive of that behavior, and slut is only slightly more so. While people can learn from their mistakes, first they have to look at themselves and decide that they did in fact make a mistake.

Bigtime DB.

+1 The most sensible post in this thread.

People keep telling themselves the same lies. Eventually they start believing it.

Finny how many of you compare the situation to your long marriages. It is not even remotely similar and the comparison cheapens your devotion/obligation/commitment.

Cheating? No way. If you are not married...you are just a booty call. A roommate/friend with benefits. Having a child under these circumstances (without legal protection and married parents) is tantamount to child abuse.

As stated earlier, few would buy a cow if they could get milk for free. Women are fools if they think otherwise. If he is really thinking forever...he will seal the deal. Keeping his options open should clearly telegraph his intentions (waiting to see if something better comes along).