I think you've muddied the waters a bit by framing this as a question about 'the realities of parenting' and 'how hard it can be' etc etc.
Yep all that's true, but it's also true that you CAN'T prepare for that.
So by couching it all in those terms, you're giving him an easy answer which lets him deflect from the real issue (and same goes for the many posters on here saying ahh, he's right, you can't know how hard it is until you have a baby!)
This isn't actually what you have an issue with.
You have an issue with being married to a man who is totally up for having a baby but absolutely 100% refusing to discuss how you will split the financial and free time workload when you do.
Which is basically the same as him wearing a massive neon sign which says 'I WANT A BABY BECAUSE IT SOUNDS FUN AND CUTE BECAUSE I DON'T EXPECT TO DO ANY OF THE HARD WORK THAT COMES ALONG WITH IT.'
Yep you bet he's enthusiastic. The two elements fit like hand and glove. He's enthusiastic and sees it in very simple 'we'll work it out!' terms because he absolutely assumes that there will be no hard choices for him to make.
If he was working on the principle that post-baby he'll have to rethink his free time, that he'll have less free cash, that he'll perhaps have to work compressed hours two days a week to do the drop-offs and pick-ups so that you can both juggle work hours... he would be LESS simply enthusiastic because there would be both good and bad to consider.
in his world, there isn't. It's all good man! Because all the bad... will be your problem.
Quite simply, it should be a no. Especially as you're not even sure you want to.
I'd take some of the points made on these posts, condense them into an email even if you think it would get the points over clearer, and use it to explain to him that until he really grows into an adult who understands the concept of the mental load and of how things would change if you became parents, then no, it won't be happening.
And that it also won't be a case of him just saying ahhh, I understand - and discussing mat leave and finances etc... it will also be a case of him changing, now, until this comment:
He does do his fair share. But it's the 'mental load' which I carry and which I think would increase even more with kids. That's the bit he just doesn't get.
- is no longer the case and he DOES get it and also DOES do it.
Maybe that's the first stage OP. Start pulling him up on the mental load. Start dropping the balls. Shopping, cooking, outings... stop being the one to plan, remember what he's forgotten etc.
Ingredients missing for dinner? Didn't you know we were out of tomatoes? But you shopped. It's not my responsibility to do the advance thinking for the shop. You'll have to go now and get some won't you...
All that kind of stuff.
But in a word, god no don't have kids with him until you've put the thumbscrews on here for a good year or more, and see whether he DOES step up or starts getting sulky and resentful when you truly, really ask him to give his 50%