It's A Theory

"Why are boys so fond of their penises?" Mrs. NoCo asked not unreasonably.

"Because," I answered after a moment's reflection, "it's our first and best toy. If women had breasts from birth, you'd be just as in love with them."

"Hmmm," she replied. "I doubt it."

I don't think the two correlate. Breasts are a secondary sexual characteristic.
I think dudes are fond of their schlongs because society inculcates you to believe that that bit of gristle makes you The Shit.

Beyond that the early discovery of sexual enjoyment and masturbation occurs in women as much as men. We just aren't schooled to flaunt it.

That explains sidesaddle.