Why would a guy cheat on you while pregnant and than leave you?

Basically I was with someone for 4 months than got pregnant I'm now 6 months pregnant and found out that he had been cheating on me for 2 months with some gurl that has kids I don't understand how a female with kids would not care that I'm caring his kid and he's leaving me to play family to someone elses kids I don't understand it he wants me to be civil about it but I don't understand how that would be fair to me I'm now living alone and I'm deppressed have no appetite and I'm praying he comes back I feel stupid but I don't want to be alone threw this anyone have any suggestions o also his reason he cheated was we weren't having sex but our living arangements with his family didn't give us privacy to do so and I'm a fultime worker while he is unemployed and the last thing I wanted to do was have sex can someone please give me some insite I feel lost and alone I just want this nightmare to be over. ;"(