Its Law, I tell you!

If you have high MP in a final fantasy, you are (usually) a magic user.
If you are a magic user in a final fantasy, you are (often) a girl.

Therefore by the Law of Syllogism:

If you have high MP in a final fantasy, you are a girl.
Cloud has high MP in Final Fantasy VII.

Therefore by the Law of Detachment, Cloud is a girl.

And who ever said Geometry isn’t good for anything? >D