Why do men cheat on thier pregnant gf?

my bf and i have been together for two years now...im three months pregnant already an this is my first pregnancy. ever since i met my bf i knew he was gunna be the one for me. In fact hes actually my longest relationship and im also his longest term. we've done everything together! we traveled together and even lived together. (i moved in with him, his dad and his sister becuz i was having conflicts with my parents) living together was actually pretty different... we got to know eachother alot more.. it was weird at first but im glad we lived together bc now i feel like hes the only guy i truly know..and he knows me more than ever. well anyway i only lived wit him for 8 months because his dad and i never really got along.. (like he would get into his head alot telling him that i was only using him for the money and that im not the only girl in the world) my bf knows thats not true. his dad just simply didint like me so i hated him bak... but it was really hard bc then we started to argue so much not just bc of his dad but many different things like lack of trust and all. the first year my bf became super jelous and was aware of everthing i i was doing, at first i took it ok bc i knew he really did love but then he started becoming so controling and realized it was not ok! he didint want me to party any more or hang out with friends. (btw reason for his attitude was bc the first year he cheated on me but only kissed the girl, but i also cheated on him that same year and only kissed the other guy so i forgave him and he forgived me bc we really wanted our relationship to work out) but tring to work out our relation ship was really hard due to what we both done that first year. we both had trust problems alot so i gave up everything for him! my friends and partying beacause i really loved him. hes done so but so much for me i feel like if ever lose him i will go nuts! so we've been workin out everything that year but when the second year came i moved out of his place bc i simply could not stand his dad. what really bothers me of that man is that he judges me and still gets into my bf head. if i try to judge him my bf gets super mad and starts defending him. so yea i def moved out to live bak with my mother again because i was getting tired of that. i felt like my bf was not taking me serious any more or putting me as his first priority.. everything was always his dad now.. (of course cuz thier both working pals). what i didint get was why didint he leave me then if he always listen to his father... but no he never left me! of course i didnt bc i love him to death and im willing to do anthing to save my relationship. so three months ago i found out that i was pregnant and bf was not happy at all!! (i was super happy but scared) he even gave me the option for abortion and that he was willing to pay any price... i got super mad (almost agreed bc i didnt want him to hate me but then realized that i would be so stupid if i did bc im being selfish only thinking about myself and him, i really wanted my baby). so i refused and told him that with him or not im gunna have this baby!! as the days passed he changed his mind and told me that he wanted to be together now as a family and started to have more love for his baby inside me... but latley due to the hormones i been B****ier than ever to him and we would fight even more.!!!! but i would always try to remind him that i love him and to hold on tight. but what i dont get is that even if i try my best never to flipp out he takes away the little patience i have for him!!! he flipps out of no where sometimes. sometimes he'll be straight up wit me and tell me that he needs sum time and space for himself when ever i visit him. He harldy vists ME! everytime we go somewhere hes like always acting weird as if hes in a hurry or something but he tells that its only bc hes tired. hes been more rude to me than ever and hurts my feelings alot but i try to blame it on my hormones. but whats weired is that when he does end up hurting me emotionally he has no sympathy what so ever. one night when i came to his house to visit him didint even last twenty minutes thier bc we started to fight and so i left crying, when i left his drive way i thought he was gunna go after me so when i got home i was actually even waiting for him!! saw no sign of him and decided to call his cell. called many times but no answer then i find out that he wasnt home (so i thinking to my self great hes prolly on his way here to see me) but no, four hours later he comes to my house and i asked him where he was, he was honest wit me and told me he went to go vist a friend of his he just met a friend that was a girl who had her own apartment and is three yrs older than him. i asked why and what was he doin there and he that they where only talking and eating dinner!!! um hello I DONT THINK SO....... could my guy be cheating on me all bc im pregnant.!! guys please give me hones