The key thing here is that it was panto - not a normal theatrical production.
If it was any other theatrical show - a musical, play or whatever, then yes, I think latecomers should have to wait as it can be disruptive when people come in and out and it's a more mature audience generally. At a show like that I also get annoyed with latecomers and people getting up and down.
Panto, however, is a different situation. Panto tends to be something of a free-for-all - lots of kids who are going to be up and down to the loo, screaming and shouting throughout most of the show. I would have no problem with you bringing a baby (your choice) - I saw a lovely baby at panto the other week who was much better behaved than most people there and seemed mesmerised by it all. I do think, however, it's a bit unreasonable and somewhat unrealistic to go to a panto and expect the same level of quiet and strictness of rules regarding latecomers as you would at another production. There was a high probability you would have to get up and down a fair bit anyway to let people out to the loo and it was always going to be a bit of a noisy crazy affair.
While disruptions generally are annoying to performers on stage, in panto they're not so bothered and just roll with it because panto, by its nature, relies a lot on audience participation. I say this as someone who's best friend has performed in everything from west end to panto and in 2 of the pantos he's done the performers have come running into the audience chasing each other and climbing over everyone! Had you been at a different panto - you could've had that to contend with while your newborn was having a snooze!
I went to see my friend in panto the other week and had a kid swinging off the back of my chair and all kinds going on. I go to the theatre a lot and I only really go to panto to see my friend in it as I prefer musicals and plays... but when I do go to panto I accept that the usual audience rules do not apply and it's going to be organised chaos at best.
If the gentleman hadn't caused you to have to stand/let him past... odds are on it would've been someone else sooner or later.
So if it was another type of show - then yes, I'd agree with you re latecomers etc... but with panto, I think you just have to roll with it a bit and not be quite so unrealistic in your expectations.