Age Is Just A Number, Unless The Number Is The Same

Help me understand this logic. In order to prevent your stepdaughter from being sexually active with a boy her age, you try to marry her off to a 23-year-old man? Huh? I just don't get it. Of course, after family told police, the guy got arrested, but...I just can't wrap my head around it, especially when it's said that the girl and the man had sex. So...you didn't care that she had sex, just don't do it with another boy?