Two things I didn't understand
This is great horror movie, but it suffers from two great lapses in logic that are kind of hard to overlook. First, is the size of the parasites. While these trilobite-size parasites makes for some good gory scenes, it's not clear how they could have initially been sexually transferred (especially to men). Even John Holmes, the famously endowed porn star, couldn't have accomodated one of these things without feeling it. Even if they were orally transfered the first victims would definitely realized it. (Of course, maybe they only grew to that size after they incubated in a human body for awhile).
Also, the motives of the mad scientist seemed pretty off-the-wall. If there was a time that man was an "overrational" being that ignored his "animal instincts" it certainly wasn't the 1970's. Why did he feel he needed to create a parasite to make people have MORE sex in the swinging, disco-era 70's? I know the guy was a MAD scientist, but this is ridiculous!