My first relationship (that led to intimacy, not just a few dates) was with a girl who had many partners before me, some of the "one night stand" variety. I was 23, she was 21, and was pretty honest and up front with me about her experiences. It made me really uncomfortable and caused me a lot of questioning and doubts, but we really got on well, and both liked spending time together. I'm not moralistic or religious, but I guess I was one of those "germophobes" worried about STD's, as well as pregnancy, so I was a bit uptight as it was and a little surprised that there actually were people who had casual sex (naive little me!). In spite of this discomfort, I stuck it out and I don't regret it, she taught me a lot and opened me up and she appreciated my willingness to keep working and exploring.
So I can understand how the guy feels, and know how hard it can be to put those doubts and discomforts aside, so I'm willing to cut him some slack. I wouldn't make any excuses for him either; as others have said, if he doesn't learn to accept people's pasts, it's going to be tougher in the future.