Whoa! Stop! (sorry, had to say it)
I went through a similar situation with my husband, who is now my ex. I was 21, he was 22...I had just had our second son...very similar situation.
You have no idea what is going through his head right now. I don't really think that saying "go sleep with somebody else so I can" is appropriate. What if he isn't cheating on you? What if he doesn't want anybody but you? What if he is having some other kind of issue that you don't know about? How would it make you feel for him to address you with that kind of statement?
One of two things is going to happen if you use that approach, and I don't think either are good. Either (1) he is going to be extremely offended that you would even say that, think you are doing things you shouldn't be, and then things will blow up or (2) he's gonna say "ok, see ya!", in which case you are going to be hurt and things are going to blow up. Both sound kinda sticky.
Have you tried to have a serious conversation about it? I mean your whole situation in general, including but not limited to the sex thing. Having a baby takes a serious toll on women, but it takes a toll on the guys, too. Maybe he is having issues he doesn't know how to handle on his own....
Don't get me wrong, I am not saying that it isn't a problem or at least an indicator of one. I went through the same kind of thing, at it was because there was a problem. I just think that you should maybe try a more level approach to it, that's all.
Good luck