In the latter instance the word has a Latin origin, and means “man”. In the case of homosexual, the root word is Greek, and means “the same”. (Same use as in “homogenize”.)

Think about it. If the homo in homosexual meant man, straight women would be homosexuals.

(It’s this confusion, I am convinced, that leads to the phrase “lesbians and Gay men”, instead of just using the all encompassing term “homosexuals”.)