It probably has something to do with our Puritan ancestors.
They were even more sexually deprived than in the Mother Country, so sexual prudes were preferentially selected for. Their wacko religion did the rest. That was a long time ago, and few of us are actually related to the Pilgrim dudes, but cultural memes tend to be propagated, even to the point of unrecognizability. Fortunately, this problem is taking care of itself, slowly but surely. There are fewer old WASP sexually hung up people every day. They are rapidly becoming a minority, and they're pissed.

In South America, by contrast, the Spaniards weren't interested in building the New Jerusalem, they just wanted to grab as much gold as they could and go back to Spain and live like Pharaoh. They didn't bring their women, so they weren't squeamish about coupling with the Indians and the African slaves, neither of whom was in any position to say no. They weren't very keen on settling down here either, although they soon learned it wasn't all that easy to go home once they got here.

The result: in Latin America they didn't develop the strong economic and civic institutions we did here, but neither did they get bogged down in racism, fundamentalist Protestantism, and sexual dysfunction, either. And that ugly trio of American neuroses is tailor-made for exploitation by right-wing politics, isn't it? People who aren't getting laid regular can't think straight. They blame other races and nations for their financial problems, and think God will smite the unbelievers because he is their personal friend. If you are sexually frustrated, you have to channel all that energy somewhere, religion and politics are just the ticket.

Yes, I realize its overly simplified to the point of triviality, but there is a grain of truth to that analysis, trust me.