My wife got pregnant by someone else while we where married in Missouri. We have tried contacting the biological father with no response and I want to adopt here so he can not come back later on and try to take her from me. How can I do this if we can't reach him?

Interesting issue, Since legally if you were married when she gave birth, legally you are already the father,

But if you want to do an adoption, then hire an attorney, the requirements for contact or attempted contact will have to be done according to requirements of your local court.

I believe the law states, in all US States, that when a child is born to a married couple, the husband is presumed to be the father. Was your name put on the birth certificate?It should have been.

This means there is no need to adopt. The bio father may be able to come back and challenge paternity at some future date, but there is nothing you can do about. You are in kind of a legal limbo. You can't adopt because you are already the legal father. You can't stop a future challenge unless MO law puts a time limit on challenges.

I would consult a local Family Law attorney to determine your best course of action.