I believe it's the masculinity that we are attracted to, not that the sex itself isn't interesting enough.
I'm going a little jaded in my thinking here. so bear with me.
comparing a man doing a woman vs a man doing whatever walking freak of nature. the man is man and is the person that turn you on, but the object is different. hence i would suggest that it's not just man in action that turn you on, but the secondary person involved also influence your feeling.
Hence if freak don't give you the sensation, why would women do consider most would never dream of having sex with woman?
I wonder are we more bi then we think?