I'm doing more research and it says that Ureaplasma is associated with non specific urithritis in males and genital tract infections in females. So, if you're the one that has the infection you could have given it to your husband.
It might be a good idea to call your Dr tomorrow to get more information about this. It's not good for you to be stressed right now since you're pregnant.
In my opinion, from what I've read, your husband probably did not cheat on you. One of you had the infection and gave it to the other one.
From what I know it can be sexually transmitted. But, it can also be transmitted if you have an upper respiratory infection. Did you Dr tell you where the infection is, or did it show up in your blood work?
I'll do some more research to see what else I can find out about this.