Strangely enough my EXH done this twice, the first time he said it was just "kissing" I still do not know the truth about that as I wanted to believe that part, he did scream in my face towards the end of our relationship that is wasn't "just kissing", she was younger, she was having problems at work, he listened to her, she listened to him, I was too busy being moody, depressed and pregnant.
He didn't want me to blame this "girl" either, I did though, I tracked her down and went to her door, she wouldn't answer, so I basically stalked her, with my 3 week old baby in the car. This went on for months. Until one day he finally screamed down the phone at her to F@ck off, that made me feel better.
We went to Relate after this but I walked out on the session as I felt the lady didn''t wan to discuss the affair, she wanted to move on from that and I wasn't ready.
I moved house so we could really get over it and he did seem to change after that and was kinder, drug free and everything he done was for us.
His second, was/is again a younger woman, her brother has just died and she was living with someone who abused her (as he did me) so EX removed this man from the house, then moved himself in.
He spent the next year pretending to come and see the boys but asking "how can we fix this", "can you ever forgive me" and "can we ever get over this" "what have I done" "what a mess" stupidly I thought I could forgive it all, I was looking at a site which I wish I hadn't now, but may help you, "Surviving an Affair" and Marriage Builders, my EXH is still with his younger bitch now and she is pregnant with their second child, she lost the first one, she fell pregnant after 2/3 weeks of them being together, then the 2nd time, he came back here stayed 2 nights, we knew it was over so he left and moved back in with her...and the dates tie in with her falling pregnant once again. Baby due August which I hope will take up more of his time and he will leave us alone, for good.
My marriage was horrid though, he was abusive and controlling and he still tries to be but I see it as a blessing that he met her as she also seems to be quite an abusive person who likes to be controlled, they are also both on heroin, so it's a match made in heroin I say, I was torn apart when he left though, more for my loss of marriage than anything else, I would have taken him back even after she was pregnant, he said "it is fate" they lost the first baby and "baby died as he was supposed to be with us" I was weak then though and depressed and he played on that.
Now I would not spit on him if he was on fire and filed for Divorce 4 weeks ago. (Lawyer wanted to wait the year so he couldn't refuse)
Your DH needs to take more blame than he is doing, the other thing about EX first affair was even after it he was still "sneaking about" to call her each night, I had no idea, I just thought he was being helpful by going to the shops, every night, he also didn't want me to comfront her, he was very protective of her too, he said afterwards he just didn't want me finding out and didn't know how to end it without this happening? Then 6 days after the birth of DS2 he screamed in my face that he had this affair, sorry he called it a "friendship" 
at the beginning of the pregnancy and that I knew something was going on. (Which I did really, well I knew something was going on, I just had no proof, just a gut feeling)
Both afffairs though were/are with younger women who EX felt needed him, I don't think I could have done anything different after the first affair, I did ask him to leave but he refused, he said he wanted us to work this out, he didn't want to be with her he wanted to be with me and I was somehow flattered/hopeful for us by this.
If I could have gone back in time I would have made him move out after the first affair and made him feel more responsible rather than it all being about "how bad it was at home". I also would never have let him come back as he really just wasted another 6 years of my life by going off and doing it again. I never ever forgave the first affair and this was his excuse for the second.
Don't be second best to anyone Orm, you and your DC deserve better.
The reason I say not to text/call her as I did this both times and the first girl just found the whole situation hilarious (she was 19), the 2nd actually called me up, to advise me to stop texting her boyfriend (my EXH), she also took great pleasure in detailing when they "made love", they are his words he used with me for 15 years.... I almost LOL tbh if I hadn't been so shattered by it all, she then phoned me up many more times just to get to me, I ended up having to change all numbers and had Solictor draw up letters to stop them contacting me.
Thankfully, my DC are not involved in/with them as I refuse to allow my DC to go to a drug addicts house, her DC were also taken into care. Sadly my EX is too weak to go get help from a Lawyer for supervised visits so his boys miss out.
My Dc know everything, I was advised not to hide anything from them, so I don't, they know have heard me being screamed at by this OW and refuse to go near their house even if they were allowed, they detest OW, I realise this may change as they get older.
One of the funniest thing Ex said to me when he came back for the 2 nights, was "she says she understands me" "do you understand me?" , yeah I understand he is weak, controlling, bullying, abusive complusive liar who thinks of no-one but himself.
Our situations are different but the women seem to be very alike, it took me a good 2/3 years to get over the first affair and even then it still came out in arguments because I ever ever knew the truth. I phoned the B&B he moved into whilst this affair was going on (pretending to be his boss querying payments) and I know she stayed there 2 nights, so this heightened my suspicions of it being more than "friend" and "kissing" but to this day I do not know the truth, I spoke to her friends and apparently "she wasn't like that" i.e. she wouldn't have slept with him but she stayed over at a B&B wth him????
I don't know whether to post this as it's all about me, but I hope that somewhere in this story/nightmare there will be something of use to you. Such as my willingness to forgive all and make everything okay just for the sake of our children and for the 15 years we did have together.
I hope this mess ends up happier than mine but tbh for us, it has turned out fine due to him being an abusive/controlling man who will never change.
Feel free to scroll though this but I hope it can help you in some way.
EX father did the exact same, EX has 2 step-sisters, his father is also bully/controlling etc. he is Jehovas Witness Elder so this seems to make him a nice person though. I see the fear in EX-MIL though, running after his every need!