how can he possibly want to cheat!?

My SO has been kicked out of his house, bounced around, and is now a semi-recovering addict. I took him in a year ago when he had no where to go, we didn't start officially being together and using bf gf till 5months ago. I bought him all the clothes, food, living supplies he could need&i gave him my bed, he promised to never cheat on me, be he knows it hurts, but I don't think he gets that sexting is cheating too... I don't know how I'm supposed to deal with this pain I literally gave up so much in life to help him and I'm being sh*t on for having a big heart. Why can't people just love and not be so preoccupied by sex and materialistic things