That music may be "masculine" or "feminine" is an interesting notion. Off the top of my head, I'd say Beethoven's music is the most masculine of the great composers -- aggressive, rhetorically powerful, very much stand-up music. Near the other end of the continuum I'd place Debussy -- soft-edged, seductive, mysterious, etc.
As for Sibelius, I don't hear his music as feminine at all. Consider the First Symphony, or Finlandia, which both strike me as masculine, rhetorical, even nationalistic. The Sibelius 5th is more modern, shows some evidence that Sibelius may have listened to the Impressionists, and is a more subtle and complex piece of music. But the music -- even the great ending -- still seems, to me, the product of a masculine mind.
Wunderhorn, you referenced the Jungian concepts of anima and animus in your post. That leads me to think of the Jungian idea that, as we grow as human beings, we integrate our personhood -- which means we take on some of the qualities of the opposite gender. I hear more of the feminine in Sibelius's last great work, Tapiola, than I do in his earlier major pieces.