Well, not entirely true. Not all Pokemon are breedable. Some Pokemon belong to egg group - ah, I think the name is Undiscoverd, however that could be the wrong name.
This is a list of ones that wont make an egg in the nursery. These are the ones that I know of, may be more:
nidorina/nidoqueen, articuno, zapdos, moltres, mewtwo, mew, pichu, cleffa, igglybuff, togepi, unown, tyrogue, smoochum, raikou, entei, suicune, lugia, ho-oh, celebi, azurill, wynaut, elekid, magby, regice, regirock, registeel, latias, latios, kyogre, groudon, rayquaza, jirachi, deoxys, budew, chingling, bonsly, mime jr, happiny, munchlax, riolu, mantyke, uxie, mesprit, azelf, dialga, palkia, heatran, regigigas, giratina, cresselia, darkrai, shaymin, arceus, victini, cobalion, terrakion, tornadus, thunderus, reshiram, zekrom, landorus, kyurem, meleotta, genesect.