RMS was objecting to the legal definition of rape (hence the reference to it varying between countries). But if you want to switch to the moral issue, then that’s even more black and white. Is it ok for a 70 year old man in a position of power to (hypothetically) have sex with a 17 year old girl? Erm, no. Can sex in such circumstances be fully consensual? No, it can’t. So if your suggestion is that RMS was making some kind of defensible moral argument, I can’t imagine what it might be.
I happen to be English so I’m familiar with the laws in England. I don’t see how any of that is relevant. The English law has cutoffs of exactly the same kind (it’s just that the relevant age threshold is usually 16 rather than 18). Obviously, no-one thinks that any such threshold can be completely non-arbitrary. It is just as "morally absurd" for sex between two 16 year olds to be legal and sex between a 16 year old and a 15 year old to be illegal. But unless - like RMS - you fail to recognize that age is a factor in consent, you have to make a cut somewhere. Apparently, from RMS's point of view, the most troubling feature of such laws is that they may prevent 70 year old Profs screwing 17 year old girls.