tricky.
I always try to give people the benefit of doubt, so here it is:
he should have told you that the plans have changed - bu I guess he not only lost track of time, but perhaps thought he wouldn't want to bother you on your night out? (or ring you and embarrass you while in the theatre?)
he probably avoided you because he feels guilty - and I think you are right to be cross with him for not doing the task he said he'd do. it is annoying, especially that he'd offered to do it.
I can understand your worries about the future/preparing for baby/money, but believe you me he's just as worried and probably needed to either have a chat with that person or have a little escape from thinking about it.
we all need that sort of "break", I think you will both realize soon that these spontaneous staying out events will stop when baby's born, and after wards you'll have to plan and life will be revolving about baby.
it is hard to give up the freedom to choose as you wish, so maybe he's panicking - having a baby doesn't mean you'll never go out again, bu it certainly feels that way.
on the other hand I totally understand your frustration at his irresponsible behaviour, and I think an apology is in order from him.
budget is tight as you say, but I suggest you try and agree on a set amount of money each week/month that each of you can freely spend how you wish - you do need a bit of indulgence every now and then, so it's better to calculate it in.
and about baby stuff - I know you probably want the best and newest things for him/her, but babies don't care whether they are dressed in Primark or Gucci!
I'd look into buying stuff second hand or even borrow things, cheap and cheerful and only buy a few special things.
Whatever you save this way put it towards a "Baby Found" so you'll have extra money to spend on childcare or a cleaner to help you out, or after clubs or piano lessons or tutors later on.
I hope you can forgive each other soon, there's no point in arguing, but you must communicate!
And he should take a bit more care to not upset you, it really is damaging to baby if mum is stressed!
I hope this helps