I think rivalshipping is because rivals just tend to have more chemistry and have more developed relationships since you spend so much time on their motivations for being rivals. If the rivals are a male/female pair, people immediately assume there is some sexual tension there, to the point where the subtext can be argued as canonical (in a literary analysis sense).

It's why I always get so annoyed when antiyaoi types shout "but they hate each other, they'd never sleep together!" And that makes them different from male/female rivals... how?