It took two to get pregnant but only one to lie about the birth control.
I do not think that this guy should have married her just because she was pregnant. I can even see what you are saying as it relates to the fact that yes life would be easier if people only had sex with people they intended to marry.
However, I do not think that should mitigate the fact that this woman was so deceptive all through the marriage. She was not even to be totally honest on the stage and wou ld have denied what she had confessed if Dr. Phil had not confronted her with a video of her statements.
All of that aside, it would have been easier if he had just continued to wear a condom. However, he cannot take the blame for trusting her with the truth when he asked her whether or not she was on birth control.