I wouldn't stay even if it wasn't his kid.

Once a cheater, always a cheater!!!

Also, his family hooked him up with her? Classy.

Yeah, you may have exposed a plot hole. He can't marry the other woman if he is married to her. 
 

OP, if you are real, leave him. If religion is the issue, i think even most priests would understand. 

How could he marry her when he's already married to you? 

You're putting the cart WAY before the horse.

He doesn't even know if she's PREGNANT. Even if she is pregnant, there is no guarantee that the baby is HIS. Even if the baby is his, there is no guarantee that he'll get any sort of enforceable international custody that would allow him to bring the baby to the US for visits versus him having to go to Cuba (since the likelihood is that the custody case won't be heard in a US court). Even if he gets custody, there is no guarantee that this woman won't fight him in court every chance she gets and bleed him dry financially and emotionally.

You say you won't resent this child, but you might be very surprised by how you feel when your H is paying out so much for child support, plane tickets, court battles, health care, etc that you no longer have the funds for travel, to update your home, or follow through on adoption.

Let's also add into the equation that IF your H finds out he can't get custody and the only way he can see his kid is to be in Cuba, he may very well nuke your marriage, go to Cuba, and marry that woman.

As crap as this sounds, she's (maybe) giving him something you two can't, for whatever reason, have together, and that's a biological child. Don't think that his love for you will somehow overtake his love for something he can't have with you. That's not me blaming you in the slightest; that's me telling you that it will be easier FOR HIM to divorce you and saddle up with the baby momma. ESPECIALLY if he is a man who is prone to giving in to peer pressure and fear (and given that he cheated on you, he seems like he'd fold like a wet noodle).

Plus, what drove him to have sex with another woman? Do you two have an open relationship? Has he cheated before? Do you just not care about physical infidelity? Different strokes for different folks, but if you have ANY resentment or unresolved feelings about him cheating, then that's only going to fester as this international custody battle rages on while you don't get to have a baby of your own AND have the threat of your marriage ending looming over your head.

But, if you're hellbent on making this work, here's my words of wisdom from one SM to possibly another in this situation:

1.) Be disengaged in all of this and with your SK. This is your H's mess, and he needs to sort it himself. His child is his responsibility. No cooking, no cleaning, no taking care of the kid in any way, shape, or form. If you'd like a relationship with the child (and this is an instance where I'd highly discourage it because your H could rip the child away at any point that your marriage becomes inconvenient), then you act like an aunt at best. Play with him, be polite, but anything that involves discipline or parenting falls on your H. That also means that you NEVER babysit.

2.) Your H needs to make sure your life changes not at all. He doesn't get to dump his frustration at BM on you. He doesn't get to rely on your for support. You get the happy-go-lucky husband you married, and he continues to help provide a good lifestyle and trips that you've both become accustomed to. Sure, you'll have to be a little flexible with dates to accomdate his custody (your sacrifice for not kicking him to the curb), but he needs to keep providing the same lifestyle. That's probably going to be nearly impossible u less he finds a job paying twice as much, but that's on him to sort out.

3.) Put off the adoption for at least 3-5 years. Yep, you heard me right. Before you bring another child into this mess (and a child who is already having a crap life), you need to make sure that custody is sorted for your H, BM has quieted down (if she ever quiets down), and that your H isn't trigger-happy in regards to ending your marriage. That's a sacrifice you'll have to be willing to make if you stay with your H.

4.) Couples therapy is a MUST. Not a "maybe". This is too raw for you to fully accept what has happened, and you'll need help to figure out why he cheated, why he didn't use protection, why he wants to divorce to be with BM, etc. You will have to rebuild your relationship with him, and you can't do that alone.

This is going to be the toughest thing you'll likely ever go through, and it will go on for years, not months.

If he plans to marry her, he'll have to divorce you. Bigamy is illegal. You are not a stepparent if he divorces you. And why are you staying with a man who plans to marry someone else? Even if you want to forgive  affairs, you are forgiving him wanting to marry that woman?

The timeline is ridiculous. Married 11 years, he cheats and three weeks later, there's a baby on the way and he is considering marrying the woman in Cuba or fighting for full custody and OP is trying to figure out how to SM from afar.

Nope.

If he's so hung up on "doing the right thing" he shouldn't have been dipping his wick outside of his marriage.