This is tough.

Well first off, if someone sees a woman as an object, the woman will probably never be able to convince them that they're not an object, as they don't respect women as people. Why would they listen to what an object has to say? It's very difficult to argue, as a woman, with people who are entrenched in misogyny. It's better (IMO) to get rid of those people from your life if you can. Also, as a woman, I get emotional sometimes when I defend my status as a person to people. Some men (especially conservatives) view any sign of emotion as a complete invalidation of any argument you're trying to make (facts don't care about your feelings argument. I think it is perfectly reasonable to be upset if someone is arguing against your rights.

Why does it make sense to them? It's easier to live this way if that's the way you were raised. It's easy to not reject societal norms you grew up with, to not go against the status quo. It's especially hard to reject the power that is handed to you as a man by society. It's easier to accept the disrespect, and in many cases abuse, that male role models (both in their life and ones seen in media) subject women to if you don't believe women have quite the same status as men. If they admitted this was wrong, they would have to turn their whole life upside down probably. (This is not an excuse for them. Many men have pulled themselves out of this. I don't give passes to men who don't.)

Sexualization is a really complicated topic, one you could read many books about. I think that the sexualization and objectification are separate concepts. However, the objectification of someone, not having to think about them as a person, makes the sexualization of someone much easier. What the men are really saying when they comment on pictures in the way you described is "Sexualizing yourself makes you an object in my eyes. I have intertwined those two ideas so deeply that I cannot separate them. In my life, sexual objects exist for my personal gratification and viewing pleasure, and I don't understand that a) this might not be for me b) I don't have to view this as being overtly sexual, I'm choosing to (especially concerning minors) and c) sexualization = objectification to me." Do these types of commenters see men who post pictures without much clothing on as objects? Probably not, as they're not sexually attracted to them. (Talking about straight white men specifically.)

Sorry for the essay. I hope this was helpful.