I agree. Other pokemon that evolve with a Moonstone can still breed after all (Clefable, Wigglytuff). Even Nidoking. If I remember correctly, however, that decision was made back in GSC, when rumors were all over the place about the introduction of breeding a Nidoqueen and a Nidoking to come up with some super-powerful pokemon baby. Never happened of course, but I remember some of my friends back then talking about that.