I think if you decide to stay with him, you two would do well with some sort of couples counsilling. There was obviously a reason he cheated, something in the relationship jsut wasn't there, you need to look at and fix that. Maybe it's just a matter of spicing things up sexually or maybe it's deeper than that. But acting like it was just some freak thing he did, it was all him, or blaming her 100% is just going to set him up to do it again because the real base problem is still there.
That aside -- if this woman decides to keep the baby they have, it will change things for you guys as well. Are you prepared to share him with a child? And another woman (to an extent)? He will have to pay money to her, and maybe decide he wants to be involved in his child's life. Which means also being tied to this other woman, having to see her and communicate with her and be civil with her. Maybe bringing the child in to your life. They become a package.
Stuff to think about.
as far as the pregnancy issue:
well, even if the girl does keep it: yes he will mostlikely have to pay child support but its not like he has to date/marry her.
think of this as strengthening your relationship.