I think of
Marion as a female name and
Marian as a male name, but it's flexible.
They're both borderline unisex to me, since I've seen each on both sexes, but
Marian suits a man marginally better, in my opinion.
Marianne is the girl name; and
Marion on a man suggests either a respelling of
Marian, or a surname.
I can deal with either name on either sex.. neither is particularly "masculine" or "feminine" seeming to me.
- chazda