Stephen Collins -- Fantasies About Oral Sex with His Own Child ... Wife Says

Stephen Collins told his wife just before she gave birth to their daughter ... he was happy it wasn't a boy because he wasn't sure he could resist having oral sex with him ... this according to the wife, but we're told Collins says it's a flat out lie.

Faye Grant sent an email to Collins last year -- after he disclosed he had molested and/or exposed himself to 3 young girls. In the email, she wrote:

"The comment you made just before I gave birth to our daughter when you said you hoped we didn't have a little boy, because 'you just didn't know if you could keep his little penis out of your mouth' was indication enough that you were sick ..."

Grant goes on to regretfully say, "I should have followed my gut then, and then again 14 years ago, and kicked your ass to the curb."

A source close to Collins tells TMZ the allegations in the email are "absolutely untrue." The source goes on ... he never received any such email from Grant and she never uttered a peep about such allegations during the entire divorce proceedings.

Collins' audio confession -- obtained by TMZ -- is now at the core of an NYPD investigation into alleged improper sexual contact with at least 3 girls.