IMO, you made it clear from the start that the issue wasn't the lesbian couple's sexual orientation as parents nor your accepting the responsibility to coach their daughter. It was their approach to ensuring that everyone including the kids knew they were a lesbian couple and that no matter what their daughter was going to play softball or someone would answer for it.
My opinion: You're better off without the headache considering their sole purpose didn't appear to be who would coach their daughter but rather "Eeeewww! Look at us, we're lesbians!" I think alot of people would have had a problem with them, too, because it's clear they weren't looking for a team to place their daughter with. They were looking for a reason to pick a fight, somewhere to make a stand and apparently they found it.
Good Luck to you. Based on what you've outlined above, I'd say you handled yourself and the situation very well in spite of it all.
I don't think you were wrong in trying to keep the focus on playing softball and teaching the children the sport.