Firstly, if you did have this baby & it didn't work with your dh and things were good with OM, he should accept anything you bring to the relationship! If he truly loves you & wants to be with you, he will have you, baby & all!
BUT
There is also someone else in the picture. It is not serious at all yet, but I am falling in love with him. I would have to say good bye to that relationship if I am to keep this child
I think you have to take the OM out of the picture, OP. You cannot decide whether to have a termination or not, based on a new/future relationship. I know this isn't your sole reason, but having OM there complicates things.
How do you know he wants more from you? He lives with his wife, his children. I would not make any assumptions about a future relationship while the man was till living with his wife, no matter how much he says it is over/he wants more from you! I can see you terminating, deciding your relationship is over & the OM saying 'sorry, trying to sort it out with my wife'. It happens. You will be left with nothing! Sorry to rain on your parade concerning the OM, but I would take everything he says with a pinch of salt for now.
Saying that, if being left with nothing doesn't bother you, ignore what I have said. I just need you to see the reality of married men who promise more. It isn't always (often) the case.
Make your choice regardless of him. Make your choice because it is what YOU want & what is best, right now. Based on how you feel about what is happening in your life, sans OM.
What would you do if there was no OM? This is the question you need to answer before you do any more thinking, imo.
Good Luck, this isn't a simple termination situation, there are many complicating factors here.
I am pro choice, have terminated. But it wasn't a choice, I just knew I had to do it.