I would endorse what they others have said. Be very very careful.
I found out 6 weeks ago that my WH's OC was not conceived in a drunken ONS, but as a result of a 10 year "fuck buddy" relationship. I was only told when OC was 2 and I went ahead with M after that. OC is now 6 and although there is hardly any relationship with OC, the relationship with OW continued until 6 weeks ago, usually whilst OWH was babysitting. I feel such a fool for having believed him through fake R and TT.
Now that WH has dumped OW, she is claiming child support having never wanted money before. Paternity has never been tested, but now she wants money for her sexual services, WH is taking the DNA test next week. There is still the possibility that the OC is not my WH's at all.
You know the situation from the beginning. You can get out now. You definitely know that OC is your WF's, you are not married yet. You can make clearer choices. You can walk away.
If you decide to stay with him, you have to define what you can tolerate regarding OC. I can tell you, it cuts me up every day and is very hard to bear. You will have this every day of your life. Your WF may have to pay maintenance, but he does not need to have any contact unless he wants to and you are OK with that. If he has contact with OC, he does not need to have contact with OW. It could be arranged through a third party (although be careful about that, as he might agree to that and still see OW).
Good luck. It is a very tough choice you have to make and only stay if you can really be sure he is remorseful reformed and devoted to helping you heal. Don't take his word for it.