Good question. I'm not sure I can generalize about RPF as a whole - my RPF fandoms were always pretty tiny and fairly male-dominated. If I had to guess, I'd say it probably shakes out the same as non-RPF fandoms overall.

If I'm remembering correctly, the impulse to go for "what if" ships rather than ships explicitly supported by canon for me was about trying to make femslash pairings happen, when they were less likely to be supported by, for instance, media that doesn't pass the Bechdel test. I'm not sure it *worked*, but that was the intention.