Nymph Physiology

How come their never been any male Nymphs in Greek Mythology? It's always the women I ain't got no problem with that! And I'm not talking about Saytrs.

Allow to elaborate more, theirs the succubus and their opposite is Incubus. And the Nymphs opposite is Saytrs. But why not Male Nymphs?

Are sirens considered nymphs, mermaids, both or neither?