Do you know the nature of his relationship with this woman? Is it a friend of his that you're aware of? Is it a strange girl you've never met? I'm just trying to get a better sense of who this person is and whether you know her or not.
I'm really sorry you saw that he called another women beautiful, but really, It's what you don't see that would be most worrisome. Does this mean that he's flirting out there behind your back, whenever it suits him? What if the women took this as a sign, and didn't care or didn't know that he was married? . Is he the type of guy that "goes out with the boys"? If so, I'd be worried. Some guys mess around and think it's par for the course. Some only. Not all. Which is he? Time to have a real heart to heart with him. When doing so yes, you have to be calm. IMO you have to be calm enough to say and mean, "if you ever want to mess around I would really appreciate your leaving before you do. Please don't let me be a naive women sitting at home while you are playing the field, If you're not ready to settle down, that's okay, I can accept that. Please just don't lie to me. And hopefully he can be honest with you. He's more likely to be honest with you if you are totally calm and don't make him feel like he has to lie, or you'll break down and not be able to function. If it were me, i would ask him to see a counselor with me, to discuss what's happening behind the scenes with him. I'm here if you need to talk. Congratulations on the new baby. Be strong. Remember how beautiful you and that baby are. You need someone who truly appreciates you and the baby ALL THE TIME NOT JUST WHEN YOU'RE HAVING SEX. If you have to move on to find it remember, that's all on him, not you. A man can't step out because of health issues that put sex on hold. That's barbaric behavior ... Your love has to mean more than being 24/7 available for his sexual release - if that's the case any hooker would do. That's NOT what good marriages are made of.
Whether you are able to have sex with your fiancé or not, because of having recently delivered a baby or not, in my opinion he shouldn't be doing anything to make you feel insecure about his feelings towards you. Especially right now when you may be feeling more bogged down after birth.
It may be best to have a calm and open discussion about this with him and tell him about how it makes you feel.
I don't think you should excuse this because you aren't able to have sex. That's no excuse for him to behave in any way that causes you hurt.
I've haven't experienced this, but he shouldn't be texting other women calling them beautiful no matter what the circumstances are. I suggest you talk to him about it and hopefully work this out.
All the best to you. :)