Why does he cheat? do i move on? ?

We r just friends. He has a girlfriend and we been talking for 3months.. we just had sex, we talk everyday, he comes sees me at wk and I go see him at wk. We hang out every now and then. He dnt give me his number because he dnt want to get caught by his girlfriend and tells me not to get any feelings for him. But he keeps calling and seeing me.. he works all day every day, he hardly has a day off. But if he dnt want problems then y cheat? Y still call me? Will he leave his gf?