How many options would you have in your area for your child to join a league?  I looked back and your kids are several  years apart.. so it's unlikely they would be in the same team would they? Even different leagues can often use the same facilities.. so I'm not sure it's a matter of never running into his EX.

Now.. if his EX holds a management position in the league.. I guess I could understand not wanting her to have control over your child's play.. but otherwise.. It may not be necessary or practical to sign up with different leagues.

Will they be on the same team? When my sisters and I played in the same soccer league (at different age levels/on different teams) we never played at the same fields at the same time (I remember this because my parents would complain that they never got to kill 2 birds with one stone, lol).  If each of my parents took one of us, they'd never run into each other.  Are you sure you'd actually run into BM enough that it matters? Is there another league (or sport) to put your daughter in? 

I don't think there's anything wrong with not wanting them in the same league unless they both want to play and it deprives one of the kids of the opportunity to do so.

Edited to add: I just looked at one of your prior posts. Unless things have changed drastically,  your boyfriend sounds like an ass who will belittle and verbally abuse you no matter what you do,  so does it really matter if he calls you selfish? Do what is best for you and your daughter. 

Do not worry about how your BF feels about it. Explain it to him and tell him it is not a discussion it is you politely letting him know.

It is not selfish to avoid the presence of toxic POS people.  BM included.