not to be too pedantic but considering that many other species have sex (some even do it for recreational reasons), I think it is a bit of a stretch to consider it to be definitionally the most human thing one could do. if I had to say what "the most" human thing one could do is, I would probably say something like technological innovation.
sure other species create tools but I'm not aware of any who have repeatedly extended their technology over relatively short time spans like humans have.