Thus, typically when a man of a higher social standing seduces a girl of a lower social standing, he does it because he does not wish to marry her. Yet he tricks her into thinking that he will marry her. This makes him follow through on his promises.
In contrast, if he is a man of lower social standing, then it would not be in the family, or girl's, best interest for her to marry him. And so they always have the right to refuse the match and make the seducer pay some monetary compensation, namely what others would have paid as dowry had she been a virgin.
Rashi, meanwhile, he notes, interprets מָהֹר יִמְהָרֶנָּה לּוֹ לְאִשָּׁה as that he writes her a ketuba, but Shadal objects that this is retrojection, for a ketubah is a later institution of Chazal. Rather, it means he marries her and gives her father the mohar. I am not so convinced by his objection, though that does not mean that I agree that this is the peshat in the pasuk.