I think what you said was fine. Answering a 14-yr old in that way is fine. The key thing is to NOT put them off or duck the question.
"I'm still married" is a very good answer. "I'm not interested in dating" is another one. "I don't want another woman in my life right now" is another one.
"That's her choice" is another good response. If you want to elaborate you could say, I don't agree with what your mother is doing or has done, but that is her choice, not mine.
