Non-American straight male here. I never even think about leaving a seat in between — that’s inconsiderate and will just make people who are looking for a seat scramble around the theater. And that’s true for sitting next to both male and female friends, really.
Also, as Scott pointed out: how will you be able to comment on certain moments in the movie to your friend?
I’m tempted to put this on the “Only in America” list … :)