are the guys at Irem preverts or something?
Way back when, some Irem developer was in either medical school or otherwise studying biology. The "'x'-type" classification system is apparently used to categorize everything from reproduction schemes to testing formats to automobiles (which really fucked up my google-fu), but in this particular context 'R-type' refers to the sexual reproduction paradigm.
Hence the penises, vaginas, sperm, eggs and uteruses in the R-Type series.
I had hoped to give a few appropriate examples of 'x'-types, but thanks to the ubiquity of the naming scheme, I'd end up with something like this:
R-Type: refers to sexual reproduction; Bentleys
A-Type: refers to the sort of people who will cut you off in traffic or during conversations; ex. your boss, the dude who stole your g/f
C-Type: refers to character-related functions or something similarly foo
My guess is that there's a type which corresponds to mitosis and one which corresponds to the parasitic reproduction scheme of viruses.