Ummm, I think the hard-dick-response-to-boobs is a learned/trained response.
Never saw an infant or toddler think of boobs that way. We learn it later, mostly because they're kept 'hidden' and 'private', so we start to desire. And they are sexualized via media. You've been programmed anon (me too!).
It's a cultural thing. Aboriginals running around with boobs showing does nothing, because they don't think of breasts that way (there are other 'triggers').