@SeekingPerspective
My solution works best for me, as between two independent, solvent adults with their own ideas of what they want to spend their own money on. If you are seizing each other up as potential parents, then I think the situation really is very different. 50:50 doesn't really work if one of you is going to have the babies, and all that that entails about impact on your body, your earning capacity, pension etc.
Yes, it has to be part of the extensive/serious discussions about marriage and/or having children and/or buying a home together, as that's a much bigger commitment that WILL need a completely different financial outlook then a couple who are dating, going away for weekends/holidays, otherwise living separately. I think there will be different "solutions" for different life stages, but it's best to have open communication about all that right from the early days. Even before talking about the finances of parenthood, you need to have serious discussions about other aspects of parenthood, i.e. do you both even want children together, your relative opinions of education types, how to nurture your children etc.
Me and DH certainly had "the talk" about children the moment we seemed to be getting serious about each other which was, roughly, after I'd known him around six months. If we'd not been on the same page about children, I would have ended our relationship at that point. It wasn't until we'd known each other for 15 years that we actually had a child, so it was very much "long term" planning, but that was exactly what we had agreed at the six month stage, i.e. that, yes, we both wanted a child/children, but that neither of us wanted one until we'd established our relative careers/businesses, travelled extensively, married, bought a home together, and that I'd be a SAHM for the first few years at least. We'd tentatively suggested around 30 years old would be about right for us and in the end it was a bit later as our marriage/home plans got delayed, but we got there in the end.
In our case it was definitely 50:50 for the first few years, but we both knew that it would change when we got married, bought our home, had a child etc. and it morphed in the intervening years during which we were definitely "a couple" but not actually married/living together etc, so rather than a specific change at a specific event, we kind of just merged things as the years passed.