Starting my reply at 01:27
Well I do think that it is a pity that you have obviously been holding on to your anger over this for a few weeks now OP (was it actually during the Diamond Jubilee celebrations?).
I think that at the time I would have whispered (crossly - because I couldn't have helped myself) to him to come indoors for a minute, and then once inside I would have 'lost it'!
I am not suggesting for one second that that would have been the best, or even a reasonable reaction - a calm explanation of all the reasons why what he did was not appropriate for that situation, and a discussion with him about how you could both try to learn how to automatically do a mental risk assessment of potential real dangers in any unusual situations, would have probably been a lot better.
I know that sounds way over the top for domestic occasions rather than business ones, but I think that as mothers, most of us will do automatic risk assessments all the time, particularly with our first babies, probably without even realising that we are doing so!
I will most likely be flamed for this, but I do think that we probably do this instinctively much more frequently than most dads do, but that is only my opinion formed by the 3 dads that I have had close contact with, and it certainly doesn't mean that I think that all men walk around with their heads in the clouds when it comes to baby and child rearing.
Going back to my first paragraph, as I said, I think that I would have been so cross with him at the danger he put our DC in, that I may have not reacted in the best way! However, I would have hoped that my strong reaction to him leaving the baby like that, helped him realise that actually it was a very stupid thing to have done. From your post OP, I get the impression that your DH is still trying to deny his lack of responsibility, or that he is no longer thinking about it at all, but that you are not satisfied that he has taken it seriously enough?
I presume that there had not been any incidents in the past that made you doubt your DH's ability to take basic care of your DC, or you would have not left the baby with him without strict instructions to stay with your DC, and to also be constantly attentive to him or her while you were not in the immediate vicinity?