First off, I am a guy, so I know how men work emotionally.
I would say that he is deceiving himself, and thereby trying to deceive you. If he says that he respects your modesty, yet is so hypocritical as to even own those posters, he is not owning up to what he really wants to do: Be sexually stimulated. I believe, that if you were to marry him and he still has the problem of needing pornographic images (yes, pornographic, images that sexually stimulate him in his lust), you will never be sufficient for him (unless you were just as immodest and always around for him to look up at); he will always desire to fill the lust of the flesh with pornographic images. I believe he has a problem with lust and that he has a great deal of work on self control to do for himself first and foremost, not to make anyone else happy, but for him self. Pornography destroys families. I've seen it happen to people I know.
If you can recognize that the posters are inappropriate, I am glad for you. This is not the sort of thing that you should just "suck it up" for and ignore. It is something very serious that should be addressed. If I was you, I would let him know my discomfort and why I am uncomfortable with it. For me personally, it would be a matter of: I believe immodesty is a sin. Passages throughout the bible show that a man is only to be [sexually] satisfied by his wife. That means, not by a girlfriend, not by a man, not by anyone but his wife. If he isn't married, that means nobody, until he marries her. If he tries to put you down, you don't have to take that and I would strongly say that he isn't worth pursuing. Standing up for what you believe is right is not being controlling or annoying. If he thinks you are worth it, he will consider what you have to say and do what he can to Not offend you. Hope that he isn't just doing it to satisfy you though.
"Also, he says that he respects me a lot because of my modesty... But if he has these posters in his room, does that mean he really doesn't respect my modesty? " - Yes, I believe such a statement can only be a cover-up, that is, something to please you without actually addressing the matter.
I wish you luck in this matter. Seeing that this answer is over a year late, I know that this situation may no longer exist, but I wish you well in you relationship pursuits. I hope my answer was helpful.