I think the problem is that it's a mix of the two so it's really impossible to say that something is all "nature" or all "nurture". You seem very insistent that your child likes "boy" things because he's a boy. While I agree that children have natural preferences, that doesn't mean that everything he does that's "stereotypically masculine" is BECAUSE he's a boy. There are plenty of 18mo old girls who naturally enjoy all the same things your son does.
If we are to go with the logic that your son likes throwing balls and whatever else because he's a boy, then it would follow that a female child who also enjoys those things must "really" be a boy too. Obviously I recognize that you don't really believe that, but that's what your posts seem to imply when you keep repeating that he likes all these things "because he's a boy" and that absolutely nothing could possibly have influenced his preferences other than his biological sex.
I think in general most people on here probably agree with you-- there are some behaviors and preferences that kids are born with, and that the majority of boys or girls prefer most (but certainly not all) of the things that are traditionally associated with their gender. It's just the way that some of your posts have been phrased I think makes it seem like your views are more extreme than they actually are (like what I posted above)
I too was surprised that nurture was not the only factor, but in some different ways than you've experienced (which is probably why I don't feel as strongly as you about the gender part as you do). My oldest child was extremely well behaved-- could attend college lectures for hours at a time with no snacks/toys/distractions, never let go of my hand anywhere, was very "ladylike", etc. I could have assumed this was due to my fantastic parenting skills or the fact that she was a girl, but my second girl was completely different-- she could climb up anything, would run away from you in a store and never even look back, etc. There were definitely "natural" differences between the two, but those were differences in personality, not differences based on their sex.
Several of my friends over the years have had extremely wild boys. While there may be some "natural" influence in what they are drawn to, their behavior is definitely influenced by nurture and the parents are constantly focused on how every single thing the kids do or say is because "they're boys". It's honestly insulting to me because my son has never behaved in the way that all these adults seem to believe is "normal" for boys. I've even had people say, "Well, DS isn't a typical/normal/whatever boy" and imply that there's something wrong with my son because he is well behaved and doesn't enjoy sports. Perhaps their sons would have other interests too, but they don't allow them into their homes because "those are for girls".