Agree you should believe and trust him if he says he isn't . My fiance and I have both had those dreams but I believe him and he is so happy I'm pregnant that I don't believe that he would cheat now.
I think everyone has those dreams. I know when my fiance does cause he will wake up mad at me lol and I wake up mad at him if I have one.
Even before I got pregnant my bf had dreams of me cheating or talking to other men... and I have the same dreams about him. I don't think it's true though. And it's definitely not true on my part.. As long as he reassures you he isn't cheating.. you should believe and trust in him.