Strange Guy...

I am a normal sexually active 25 yr old girl. I date frequently and if I am attracted will have sex without long term dating or commitments.
Early in my sexual experiences as a teen I learned that I enjoy anal sex. OK, more than enjoy. I love it and even prefer it to vaginal sex. To this point it has never been a problem. I never met a guy who was disappointed that I like being fucked in the ass. Guys usually break out in sheer joy when I tell them. Never had a problem getting a guy to fuck me in the ass until now.
Met a guy a few weeks ago. We went out a couple of times and he made a move. We went back to my apartment with intentions of having sex. We were naked, fooling around and he had a major stiffy. When I told him I wanted him to fuck me in the ass I expected the usual bi time yes! Instead he immediately lost his erection and interest in having sex period. I tried and even offered him my pussy but he was completely turned off.
First guy I ever met like this? Can any guys explain???