Op do not marry this man. If he hasn't cheated yet, he will. And, if he's contacting sex workers, or using virtual sex workers, paying anything - that I cheating in my mind. It's beyond using porn. He is also showing a clear intent to cheat on you with paid for sex.
If he's doing this now because you're pregnant and having less sex, what will he be like when you're postpartum and unable to? You will be advised not to have sex for 6 wks or longer depending on the type of delivery you have or if you have trauma. What about when you're feeling self-conscious as you recover? What if you're too tired ?
You could easily end up in a situation where he will end up walking all over you if you let this progress to marriage. Because he's gotten away with it once before. He will continue to damage your mental health, your self-esteem, and self-worth. You will end up a shadow of yourself filled with hate and resentment.
It's easier to leave now, then when baby is here and you're married. Yes, it will be hard, but you'll be free from all of the emotional crap that staying with him will cause you. Can you live being paranoid every day he's using sex workers? Watching out for 'the signs' that he is?
He will have to pay maintenance, and you may qualify for universal credit.
As for what you tell family and friends....you tell them the truth. As you've written here. They will most likely understand, sympathise, and respect your choice to not marry him. If they don't, fuck them.
YOU AND YOUR BABY ARE WORTH MORE THAN THAT PIECE OF SHIT.