Why is he only into woman with really large breasts?

See you're getting confused here. There's the mans brain in his head, and the mans brain in his penis. Now with the majority of men, the brain in the head is usually larger, but with others, such a pimps, cheats and STD carriers, the brain in the penis is larger. And that's where your problem is, you found yourself a guy with a small brain in his head and a large brain in his penis.