I know it’s been literal years, but I have my own thoughts on it!!
It’s not explicitly said in the movie, but there’s a small reference to a line in the book when Ennis and Alma are having sex and he flips her over to her stomach—I’ve always taken that scene to correlate with the mention of them regularly/mostly having anal sex. The look on her face in the movie during that scene seems that of some sort of discomfort, physical or emotional. In the book, there’s a comment that Alma makes about the way they have sex, saying the ‘way they do it doesn’t make babies’, or something close. In the movie, when a condom is mentioned, Ennis gets very defensive, so I don’t take the ‘doesn’t make babies’ line to mean condoms, but anal sex.
The line about the ‘North Pole or Equator’, in my mind, was in reference to them choosing between vaginal or anal sex.