Basically, because it was an era of more liberation than enlightenment, and a lot of guys ended up with entitlement complexes. There was a general feeling that if you wanted it, then it was good, and if you couldn't prove that anyone was hurt, then no harm, no foul, right? Meanwhile, they hadn't scraped away the thick crust of male entitlement. So if a 40something thought a 15-year-old girl was hot, and she was into it (or at least looked into it--and there was a lot of pressure to be into it), then there was nothing wrong with their relationship, right? Or if you weren't going to go so far as to say it was okay to boink 15-year-olds, you at least felt free to say that you WANTED to boink 15-year-olds.

And then the second wave of feminism came along and said, "Guys, seriously, this is shit." Some guys said, "Oh, hey, you're right." Some said, "Stop being so humorless. I'm not hurting anyone--look, she wants it!" And a lot of guys got caught between the two messages, both wanting to be MEN and wanting to not be dicks. I get the impression from them nowadays that they feel there are two versions of reality: the men's reality, and this other, socially acceptable reality that you have to pretend you believe in. They get creepily excited at anything that lets them bring the men's reality into polite reality, kind of like little boys giggling at dirty jokes at the back of the classroom. They tend to show up a lot in discussions of evolutionary psychology--you know, the science that proves that humanity evolved to be exactly like 1950's America.