Thank you for the responses. I did ask why this occured at the time, but I got different answers from both of them. She claims that she has been attracted to him for some time and that their relationship grew while I was having a difficult pregnancy. He says (like Annabel said) that it happened while I was in the hospital because of anxiety about the matter. Here I believe they might both be telling the truth.
However, I'm not sure if I can forgive regardless. My husband promised to be there for me during the intense time in the hospital. He only came to visit one time for a period of two hours saying he was busy with homework from college- I was in there for a long time. To learn that what he was doing was playing with my friend devastated me.
My friend has done similar things to me in the past. She is incredibly attractive (not that I'm not, just in a different way) and has slept with my partners while I've been on vacation, at work, taking care of my ill parents, etc. (I already forgave her too many times in the past I guess.)
At different times both of them have lied to me, so I have difficulty giving them trust right now.
I like what Annabel said about poly offering a different perspective. When you're hurting it's harder to see that. Maybe I will see it with time, but I'm not ready to forgive either of them.
Thanks for both of the responses. Honestly, it has nothing to do with me being mono and I should own that. I can accept the poly lifestyle, but you're right, it was cheating. I sort of wanted to trick myself into believing otherwise.