Why did Noah put only two of each animal on the ark and how did he keep them from mating?

Because God told him to put two of every 'unclean' beast but sevens of all the beasts considered 'clean'. Also the birds were represented in sevens. As for the mating we aren't told but there's no reason why they wouldn't have. Two Genesis 6:19 Of every living thing of all flesh, two of every sort shalt thou bring into the ark. Genesis 7:8, 9, 15 Of clean beast and of beasts that are not clean, and of fowl ... there went in two and two unto Noah into the ark, the male and the female, as God commanded Noah. This verse, God commanded Noah to take both clean and unclean animals in pairs. Seven Genesis 7:2 Of every clean beast thou shalt take thee by sevens, the male and his female. Note; Seven is an odd number and cannot be a pair. There will one odd animal left behind with no pair. More than likely these were used for sacrifice after the ark landed on dry ground, as at this point mankind had not been given permission to eat meat.