Most rape laws do recognize a spectrum: in the teens, it is generally not rape if the two people are close enough in age.

That is relevant but does nothing to explain why say Sweden put the line at 16 while the US has it at 18. It also happens to be the exact opposite for driving a car, ie the age requirement is 18 while in the US it is 16.

Personally I find the age when the brain is fully developed to carry a bit more significance, ie around age 25. Before that the brain is still being assembled and yet we expect people to make life or death decisions while driving, serving the military, have correct impulse control and follow the law just as everyone else.

There are European countries where it's 14 too!

Because that's what the law says. You can't vote at 17 years and 364 days either. (Or drive at 15 years and 364 days.)

What would a "spectrum of consent" even look like? 'First base' at one age, second at another, etc.?

* https://en.wikipedia.org/wiki/Baseball_metaphors_for_sex

The law used also used to say it was illegal for gay people to marry and black people had to use separate bathrooms. That black people are worth 3/5 of a normal person. So don't even start with that "but it's the law!" drivel, we are talking about morality here.

It would look like romeo and juliet laws that allow consent between two teens close in age.

..but Romeo was over 18 and Juliet was like 13. That's statutory rape!

It's the letter of the law and that's why the above quoted position (or see below) is indefensible?

RMS: "I think that everyone age 14 or above ought to take part in sex, though not indiscriminately. (Some people are ready earlier.)”

Just because it's the letter of the law doesn't mean we are forbidden from discussing changing it.