Is there anything unnatural about a woman in her early 20s being attracted to a 16 year old boy, a man in his 20s being attracted to a 16 y.o. girl?

Yes, very much so. It's creepy. As a 21 yo myself, I can't even think of dating people below 19 without being filled with disgust. The difference in experience is so monumental that they basically seem like kids. 16 year olds are actually kids