Silly me - I never thought about it in English. In French, I always assumed that the feminine rhymes were thus called because the extended rhyme often came from the addition of a feminine ending. (Man, this is being hard for me to articulate, but I know what I mean.)
example: dur/pur -- masculine words, single syllable
dure/pure - feminine words, 2 syllables (pronounced as such in poetry, although not necessarily pronounced as such in prose or in speech)- and both syllables are part of the rhyme

but I could easily be waaaaaaaaaaaaay off base