Think ladylush's advice is great - couples counselling would help, I agree.
It is especially hurtful and cruel if infidelity happens when you're already having a rough time. That was the case with my H too, although in my case, it was a series of business setbacks that had laid my esteem low. It was one of the biggest stumbling blocks for me in my recovery, as I know I couldn't have kicked him (or anyone for that matter) when he was down. I think this comes down to selfishness on their part - and some escapism too from the gloom. One of the things I've learned is that infidelity is always rooted in selfishness and is very often about immature escapism.
Aunt Maggie, what you're saying is that this man has had only a fortnight's break from serial monogamy since he was 17. This reinforces to me that he just isn't ready to commit to being faithful to you for the rest of his life. It might help to ask him how he feels about never sleeping with another woman again for the rest of his life - and watch his face carefully. Never having another romance again, never being wooed again? This might just induce a painful reality check.
When I said the mother issues were perhaps a red herring, what I actually mean is that it doesn't sound as though he's got anger issues towards women. I do think he's been used to being cossetted though and I suspect he did feel a bit abandoned when his family upped sticks. But that's the point isn't it? When ever his cossetting is withdrawn or diluted because of illness or just a low patch in your relationship, he seeks it from elsewhere. Might be worth finding out what needs the two women appeared to be meeting - it's often not about sex, but about adoration and respect.
I understand what you mean about his image amongst your friends as a superhuman carer - and also how he presents as brilliant Dad material. What people on the outside see is so different to the reality - and believe me, it is so different when they are your own children - for good and bad. Before we had children, our friends would have said that my H would have been the better parent - he loved kids and was broody for years in fact. I on the other hand, found children irritating. Babies did nothing for me.
When we had our own though, I fell in love with them and found them fascinating. Found patience I didn't know I had and it was the biggest surprise ever to me, my H and our families and friends. My H was and is a great Dad on the whole - but having your own kids full time, with the attendant sleepless nights and 24 hour demands is so different to looking after other kids occasionally.
The selfishness that caused my H to have an affair many years later, manifested itself in his early fathering, to some extent - and the lack of domestic support he gave me at home when the house was wrecked by having two small children in it. What I'm saying is - if someone had judged me before I had children, my "Mum" credentials wouldn't have been favourable, but my H's would. The reality was very different.
You might be the same. You're dead right that you reasoned that he was a grown man and could iron his own bloody shirts - that was always my perspective too. Except that now, as has often happened in recent times - I might be rushing to meet a work deadline and I find that H has ironed all my work shirts without asking, so as to reduce my stress. And if he's up against it, I'll do his. We help each other out and are both kind to one another now. The problem is, if the nurturing is all one way.
It sounds a bit as though your DP can't cope with putting his needs on hold for a while, when a situation calls for complete selflessness, e.g. when you were very ill, feeling down etc. Having children means suppressing your own needs too - and if he has a need to be cossetted, those needs are never going to be more unfulfilled than during the infant years.
And if he's got low self esteem issues anyway - and sees himself as a bit geeky - he's also particularly vulnerable to people who will tell him he's Mr. Wonderful.