The wife doesnt know about the baby, just about me.
Whatever you do about this pregnancy, please tell her that he made you pregnant if you know who she is. You both need STI tests and it would be informative for her to have concrete evidence (like, I dunno, a copy of the twelve week scan) that her "D"H can't even be bothered to protect her health from STIs by using a condom when he cheats. Plus I'm vindictive enough to want him to really hurt for what he's done, were I in your position.
I have no "right" answer on the pregnancy decision other than that it is your body and your choice until the time limit and it is not your responsibility to save his marriage. Do what you need for you, not him and not the baby. And he will owe you child maintenance if you keep it. I wonder what his wife will make of that?
Consider that adoption will mean rhw heartache of surrendering your child and then expecting a phone call or letter from that child from his/her 18th birthday until the day you die.
To the PP who talked about using the pregnancy to trap a man into commitment, piss off with your misogyny. If a man doesn't want kids, he should keep it zipped, get the snip, or use a condom. Other than in the vanishingly-rare case of a Section Four sexual assault, a man can only be "trapped" if he chose to risk starting off the pregnancy in the first place.