So the idea of a guy/girl friendship being completely free of hormonal input is bogus.
And I must reiterate that the prohibition here is NOT the words that are spoken. It is 100% permitted to speak to a women for a purpose such as business, asking directions, or having any essential discussion. It is the unessential, unnecessary, and especially friendship-oriented interaction that is prohibited.
Rav Moshe’s famous Teshuva prohibiting m’doraisa girls and boys being friends is based on a statement of Chazal in Avos D’Reb Nosson, and a ruling of the Ran. Basically, it says that any friendly interaction between boys and girls is prohibited. Rav Moshe ZTL continues by saying that there is no such thing as a platonic friendship between boys and girls. He points out that objectively boys should logically choose other boys as friends, – they have more in common, they are more alike etc., and not girls – and the only reason someone would prefer a female friend is because she is a female. And that is Asur.
Now Rav Moshe ZT’L does NOT mean that every boy/girl friendship is for the purpose of lust. The dynamics of mixed gender friendships are so different than same gender friendships, and the reason is because of the subtle but oh-so-obvious sexual dynamic taking place between the parties.