Excellent thread! Reading some of these excuses has lifted me from my early morning mooch and pity-fest (DS was away and didn't get back until evening).
My ex's excuses covers similar ground to above:
I should learn to put my partner first before my child (DS became 'my' child, not our child; ex was living a lifestyle where he flitted over every few weeks to some place or other on business, leaving me with DS and no family around, so I kind of guess I had to put him first?!; ex gave me this tidbit as a bit of advice for future relationships, so you can guess which pedestal he is on in his own house/in new set-up with OW/now preggers wife)
I should never have gained weight after having a baby. Conversely, I should have done more to lose the weight after having a baby (ex is about 5ft 8in and weighs around 17 stone).
I should never have started watching trashy TV programmes (I liked to watch 'Come Dine With Me' and 'Coach Trip' which I had taped and would not watch if ex wanted to watch something else and we liked/watched a lot of the same progs like US TV dramas, serials, films etc)
I should never have gotten PND and I seriously should have snapped out of it a lot quicker (funnily enough, ex's OW/new wife is now pregnant, so she has been forewarned. Ditto for his advice on not gaining weight post partum.....
I should have been more into his interests (note: he had no interests bar watching TV. He wanted to try Tai Chi - I found him a class nearby and he found an excuse not to go; he wanted to try fencing - again, found him a class and he never went. He of course, never expressed any interest in my interests...).
We didn't have enough sex (that would be inbetween me solely looking after DS who didn't really sleep - and still doesn't, ex being away on business, me being knackered.....we were trying for a second child so his no sex excuse was a bit weak).
I changed after having a baby; we weren't able to go out like we did before we had a baby; we didn't have a gazillion holidays to wherever we liked like we did before we had a baby.....
Those are the ones I can remember.....ultimately, OW/new wife is welcome to him (my mooching was more about missing DS and wondering what the future holds for me). And I sincerely hope that she has been taking notes on how to please ex and not make the same mistakes I did, especially as their baby will be due any day now (so much for ex running off to go and live a live of freedom that I had denied him by shackling him with a child after 8 years of such freedom 
). She has already agreed to never request a move back to her home country (as ex couldn't bear to leave our DS) and ex has supposedly told her that if I move away from current town they would move also (blimmin' hope not!)....Yep, he is the catch of the century (i.e. he has a few ££ knocking around)...!
On a serious note, totally agree as to why these men/women cant just say 'lets sit down, I don't think I feel the same anymore, blah, blah'...unless it really is because they are that scared of being alone that they have to have the next one lined up? 