We are two people in love and about to marry. He proposed to me but before I gave him an answer, he said, "I have a child by my ex-girlfriend. It was a mistake. I did it when I was a teenager, but now I love you and I will never do that mistake again." I don't have any problem with him having a child, but I really want to know is if it is considered adultery if I marry him because he has a child with another woman?
Adultery is when people who are married have sex with someone they are not married to. He wasn't married to his girlfriend when he had sex with her. That is fornication. Thankfully, he understands that what he did was wrong. I suppose he told you because he has taken responsibility for what he had done and is involved in the child's life and upbringing, which will impact you if you marry him.
His having sex before and being a father did not make him married to this other girl. Marriage is a covenant between a man and woman (Malachi 2:14). So marrying him will not make you an adulteress.