Don't worry too much as there are guys out there who would pay for that level of appreciation for their efforts!
OK it's not the average response you expect from the lady in the bed but on the other hand I guess it is something interesting to write about in the diary.
I hope he wasn't performing oral at the time?
Your boyfriends correct response should have been "wow, did I make you do THAT!"
He should be pleased and not disgusted - pooping is a fairly normal bodily function after all and it is only current social conventions and taboos that make it unacceptable to share it with your partner...
Other times and other places pooping used to be a shared convivial and almost social occasion...
He certainly wouldn't be suitable for a career in medicine...
Might be a good idea in future to try and empty your bowels before a session like that and if that isn't successful make sure you protect the sheet and have some wetwipes and kleenex handy.
I have heard of this before and is not uncommon in the problem pages of the sort of soft-porn magazines that will print letters about such things, although I have not experienced it and I don't know any who have, I do know it has been on the mind of some of the women I have spoken with - so I think pooping during orgasm has been a close thing for them at some time...
It has happened - you pooped in bed! You cannot make it unhappen and it isn't going to go away (you did clean up? its not still there is it?) so there is no point worrying about it. Having an orgasm is a bit like being drunk - you aren't really in control of your actions at that time and cannot really be blamed for whatever you do while in the throes of orgasm - some women scream and some use foul language, some get really violent with the strength of ten men and some crap the bed - so what?
The only way to put this behind you is to see the humour in it and if you can't it will haunt you for ever...
If your boyfriend can't put his shock and disgust behind him it is entirely his own problem. You have nothing to apologise for and little to be embarrassed about!
One final thing - how about having a disclaimer or health warning tattooed somewhere prominent?