Forensic proof is long gone. Also, when you are incapacitated to the point of being unable to give consent, it's easy for a person to have sex with you and not incur any bruises/scratches/scrapes, which would be indicative of a rape/putting up a fight. I don't find it hard to believe at all that back in the 60's, when "date rape" wasn't even a term, that people would be afraid to speak up. It's similar to child molestation.....the lay of the land (no pun intended) back then was to remain silent.
ETA: I'm just adding this as my personal experience that may or may not have happened to one or more of these women, and I'm still loathe to provide this amount of personal information about myself. In my case, I did not even find out until 3 weeks later that sex had occurred. I was assured by this nice man, the next morning when I woke up with no recollection of the night before and barely clothed, that nothing at all had happened. He was charming and reassuring, protective-like (much like I'd imagine Bill Cosby would be). It wasn't until 3 weeks later when we were HIKING that he casually told me that yes, we had sex that night. I didn't even believe him at first since (thought he was kidding) because I thought he was being honest the first time, which was the first night we met. His excuse was that he thought I was being "demure" because I was embarrassed that we'd done that at on the first date and he insisted I was "very into it." I didn't believe a word of it and never saw him again. Now, how exactly would I have gone about pressing charges against this person 3 weeks after the fact when I put myself into a precarious position, consuming alcohol on a first date? I'm not going to go into detail about what led up to that night/where we went on our date, but it was basically a typical date where cocktails and casual conversation were involved and that's it. At least that was all it was supposed to be. Until I couldn't remember anything after a specific period of time (where I went to the bathroom.)