My experience is that some straight guys have never learned to interact with women as just people. They grow up, have friends of the same gender, and then hit puberty--and then, and only then, do they start interacting with girls. It creates a mindset where hanging out with girls is only associated with sexual desire. So if that desire isn't there, they just don't do it.
I have a few straight guy friends who seem to really suffer from this. IMO, it's something a person can overcome. It's just a psychological association that needs to be broken.