I'm female and I have experience being in a relationship with someone at your age for the same time frame. The guy I was with when I was 24 was the same guy I had been with for 9 years up until that time. We were eachothers "first" sex partners. Is that the case with you too? He was very curious of what else was out there. He too was always interested in other girls we would see while out and pictures of girls, all very thin and young, (no internet back then) and he wanted me very often as well. I always just figured it was his age. He was 25 at that time and very horny most of the time. I just thought all men were extremely sex crazed around that age so never thought much about it.

Maybe you could contribute some or most of it to his age.