Ah yes gender roles are clearly decided by choice of sex position, but helpfully as we all know this is generally a fixed thing per person per realtionship. /s
I mean it makes only a smidge less sense in the grand scheme of things than going by whose leg-coverings are bifurcated. Which in turn is only roughly about as awful a metric as, in a boring heterosexual relationship, going by who has which plumbing... But still.