This is because Metagross and Lunatone have no Genders. To breed Pokemon they must be in the same egg group and 1 be female and 1 male. Metagross is only capable of breeding with Ditto he shares this trait with Magnezone, Bronzong, Claydol, Cryogonal, Electrode, Golurk, Klinklang, Lunatone, Solrock, Porygon-Z, Rotom, Shedinja, and Starmie and most of there evos. Manaphy also can only breed with Ditto but that makes Phione not Manaphy.
