First I need to say, I don't think "your ancestors did it" is a good defense. Probably all of us have horrible crimes in our ancestry, long forgotten, and it doesn't give us a pass to repeat them. I find "this used to be normal" or "I can find some country where this is OK" similarly unconvincing.

But something about this:

> although I think it ludicrous when we arrest 18 year olds for having a relationship with 17 year olds

I am not an expert but I don't think this happens. Long ago I read somewhere that US age of consent laws (which vary by state) carve out exceptions if one of the people is only slightly above the age, or apply a different standard for young people. So for example a couple of two high school students where one of them happened to have a birthday before the other wouldn't be forbidden.