Boys have a penis, girls have a vagina. Seriously, I think the other aspects of a relationship are where the compatibility is most important. Sex takes care of itself if you love a person.
You have a pee-pee? She has a va-gee-gee? You're compatible. SPARKS? Are you sure that wasn't AstroRocket's electric prod?
Yes I'm quite sure....I think. At any rate if you're not compatible you'll know because you proably broke up all ready.
Sexual compatibility for any given session is derived from this simple equation: C = (NFO / NMO)(mean of TMO / mean of TFO) Desirable value of C >= 1. I recommend taking the mean value of C from a minimum sample size of 30. Please note, however, that my theory does not posit spontaneous orgasms (Figure out the terms yourselves, shouldn't be that hard )