Not sure this can help as what I read was your husband just did a casual talk with the girl and it was just because he needed somebody to release the anger after the fight with you. But somehow it makes you down so hard. So I was thinking that maybe the deep sadness comes from the pregnancy, you know...unstable emotion, hormones fluctuation, etc (well I've been pregnant once before )
Dear...your task now is not planning to cheat back...really...the important thing is your baby and your health condition. You have to be super-fine when pregnant. Just to avoid a chance of pregnancy or labor complication in future. And your baby should be born healthily (physically and mentally) as well. So if you are angry or sad too long, he may get the impact from his mother. Perhaps being a bad temper boy or low self esteem one. Or maybe experience a hard labor too.
And maybe you can see the good side your husband did not mention any bad words or angry or punch or kick (ooops... I watched kung fu panda too much ) you when he was busted. It means he still considers your pregnancy and his son.
So he might just an ordinary man who needs to release his emotion.......somehow the way does not match you.......just talk to each other over a cup of hot tea or chocolate on sea shore and watch the sun set will you, dear?