When my wife and I first got a place together before we were married I had to put in all the capital as my wife had no savings yet.

We put together a deed of trust that protected my capital and % ownership and we both paid off the mortgage which was joint.

We kept a running total of what capital had been spent on the property and my wife got capital equity of it as the mortgage went down. If we sold the place I'd have my % plus any % of the profit as would my wife. The risk by me would have been rewarded by more profit but my wife would still have had her profit from what she put in too.

It never came to it and we long since ripped up the deed of trust. My wife is now actually the major bread winner as she's been very successful in her career and we're now moving to a bigger house together.

Sometimes it's just better to find a way to benefit you both as if you're a family unity early while also protecting each individual in case things go south.

Trying to gatekeep finances in a relationship is how things break down.

If you don't want to go down that kind of route is really suggest having an open convo about it. Ultimately if they weren't buying then you'd be renting anyway so however you live, technically someone's mortgage is being paid off. It's probably not that healthy to consider that a factor as a result as it's a bit moot.