In practice I think there probably some level of wrongness in most cases where a 20-year-old has sex with a 13-year-old.

I suggest that, due to the anchoring effects of formal law, there exists some combination of ages such that you (and probably the average German) would say this and the average Spaniard, including hypothetical-you who grew up in Spain, would not. It may not be precisely 20 and 13, but I strongly suspect that such a pair of ages exists.