How come men cheat on their pregnant wives/girlfriend?

I'm 6.5 months pregnant by my current boyfriend. I already have two other kids. Anyway, I quit my job recently because it was too strenuous. So my boyfriend and I agreed that I should stop working. So now his job as a brick layer is picking up and he's paying all the bills. We've been together for about 11 months and broken up about 6 times already. I found out 4 months into the relationship that he was seeing one of his old fwb's. In fact, I know he's been with her before we even met. However, he thinks because he pays the bills he should be able to do whatever he wants. So a couple of days ago we had a fight he said he it was my fault for getting an attitude. I haven't seen him in a couple of days and I heard he 's staying with his cousin. Friends of mine said they saw him and the same girl he's seeing on the side together leaving a motel. It's killing me to know this. I feel like I'm trapped in this relationship. I have no job and he pays all the bills. It's almost like I have to accept the fact he has a mistress on the side. I'm 24 and he's 27 (cuban).