Quote Originally Posted by Shining Espeon
If you're still having doubts about Masamune's gender:

[IMG135]http://www.serebii.net/anime/pictures/houen/395/AG159.jpg[/IMG135]

Evidence enough?
Plenty of evidence, even if Masamune was a female in the same hot spring as Ash and Brock we'd see her (er... him) wearing a top of some sort.