I.B.
Hi, I just came across this thread, thought I'd share my thoughts...
I was a single mom for 7 years and the biological father wasn't involved. He lived two states away and had all kinds of issues (legal, health, drugs, etc). I told my daughter that she had a "biological father", but he had some problems and couldn't be around, but that someday I would meet the right person and get married, and then she'd have a daddy. Thankfully, that's exactly what has happened!
Anyhow, I think once she asked what kind of problems her bio father has, and I just said "health". I tried not to bad-mouth him, especially when my daughter was little, and I think that's important.
Good luck!