The child's mouth and your nipple are both things that are already there. This, automatically, makes your point null and void. However, I will further prove that your original statement is wrong. Just as you believe the male nipple is not suppose to feed a child, the penis is not suppose to go into a mouth. It was meant for two things, and two things only. Urinating and producing off-spring. Blowjobs do neither of those, and as such, are not natural.
My nipples usually don't give out milk when I try to feed kids with them.
I can't squirt milk out of my breasts, but I can shove my cock in someones mouth, therefor I'd have to do something unnatural to give myself the ability to squirt milk out of my breasts.