Why is he so against me and him?

This boy and I have been having an on and off sort of relationship over the past year. Im young but old enough to know this sure is something. He's perfect to me, but he has girlfriends when he acts like he likes me... and its confusing. We shared a few texts last weekend... and they were dirty texts, and during that night he had no thoughts of his girlfriend. He texted in the morning explaining how sorry he was and that he'd 'cheated on the perfect girl' and all of that... but i didn't see it that way! It was one of the best nights of my life, as bad as it sounds. But ever since the first time he went off of me hes been so adverse to us. I called him 'Dear' a few times and he always says 'Ive got a girlfriend' but if hes so determined not to cheat on his girl why did he send me those texts last week? now he wants nobody to know about it... what should i do?