Yes sexting is cheating!
Any man who's mind and imagination is on another woman is cheating.
It will start out 'emotional' but it always ends up 'physical'.
If a man (or women) feels the need to 'connect' with another, it shows they are not commited in their relationship.
No matter what response you get though, you will keep defending his actions. You obviously want this to work because of the trouble you've gone through to get to this stage.
You are not enough for him. He didn't want a baby. He's 'stuck with you'. Have some self respect and walk away. You will be an even bigger laughing stock, when people find out he's shagging someone behind your back. Note I say when, not if.
I'm sorry that is harsh, but that is the reality. His adventurous sex life has nothing to do with this. If he loved and respected you he would not be looking elsewhere.
If you think this is bad, wait until you are sleep deprived, trying to lose baby weigh, you don't want sex because you are too sore or too tired, you spend your days covered in baby vomit, and just managing to shower is a good day! Fingers crossed you won't suffer from PND, because if you do, his behaviour is going to destroy you.
Just pack up and go now, he's not going to change, and stop making excuses for him.