HaLeiVi: If he hated the first he could (and should) of divorced her. Since he didn’t, clearly he loves both (or all three, etc.) “
The Torah is not speaking about hatred. It is talking about a man who simply loves one wife more than the other. If the Torah meant to talk about real animosity (and it DOES deal with that), it speaks to the issue of a man finding something “unseemly” in his wife, and therefore should give her a sefer kritut (GET). But when it speaks about the isha senuah, it is not what we think it means, i.e. that he actually HATES her. He does not hate her, he just favors the other wife more. So the Torah is teaching us that in such a case, if the senuah has the first child (a male), THAT is his bechor, regardless of how he feels about the mother.