When a child comes along, the emotions involved become even more irrational than they do in romantic love. People can be whatever sexual orientation they wish, hold whatever politic/religious beliefs they want, live where they want etc. but how a mother feels about her child (whether positive or negative) just cannot be controlled in ANY way whatsoever.
It sounds like the OP would definitely not like the situation that might arise. Best to take a step back. But another thought though: do you think maybe your boyfriend's other partner is actually feeling a bit broody herself? She might be feeling like she wants another child, and feels that your plans to have a child would make that impossible. This is something your boyfriend needs to ask I think.