I really don't know how to reply in a nice way....
Your partner is a major loser. He was caught flirting with a 15 year old and not only did you stay with him you got pregnant to him. On top of that when you first found out you were yelling at the 15 year old rather than your husband. There is definitely something off about that. Again you're going down the track of blaming the now 17 year old instead of your husband. He should've flat out told her not to contact him and blocked her on Facebook.
If a person is a cheater they will always be a cheater. If the only way to keep your husband faithful to you is to go and scare off other "tempations" then you need to consider the type of relationship that you actually want and you want your child to grow up with.
Personally I'd run a million miles but that's just me.
Good luck whatever your decision and I hope he's come to his senses