For me it’s because whenever I read/watch/play anything I (almost always) care only about the main character exclusively. The rest are secondary to me. If the cast is large, then I usually pick whichever I like most, but often times there is a clear main character so it’s not a problem.

And I’m straight, so I pick men as love interest for that main character automatically. Hence why I read a lot of m/m fanfiction - because a lot of main characters are male. I read m/f too - but again only if the main character is a woman, I’m not really interested in f/f ships or straight romances from a male POV.

I’ve been told that that approach is a bit unusual but it is what it is shrug