FYI, Zendesk was a "pre Point Nine" angel investment from me that I made in 2008, but I wasn't aware of these discussions among the founders. Maybe they took place before I joined or they wanted to keep them to themselves.
Well sure, I agree it doesn't have to. But as this thread has shown, it's a contentious issue demonstrated by:
(1) furious downvoting & upvoting of jacquesm parent comment,
(2) many comments in this thread from opposing sides debating it
(3) chapter from Zendesk founders' book discussing the uncomfortable tensions it caused
Therefore, saying "paying one founder more than another can cause conflict. But it doesn't have to." -- sounds somewhat naive. To be clear, I'm not saying _you_ are naive. It's that your sentence sounds like it brushes the human complexity under the rug. (We can also say "paying everyone equally also can cause conflict, but it doesn't have to." -- a kind of tautology that's true but doesn't really inform us.)
Also, it's important to stress the timeline of the startup:
It's one thing for the startup to not exist yet and the 3 founders are just sitting around a dining table and agree that 1 founder should get more salary because he has kids.
It's a very different situation when the startup is already up & running for a year and the 3 equally paid founders are living on cheap ramen to keep expenses low and make the struggling company survive. If at that point in the timeline, if one founder asks for more salary because his wife is pregnant, don't be surprised if the 2 other founders will react very negatively. It's human nature.
Recruiting a parent with existing kids to create a new startup is a different dynamic than increasing a partner's salary because they have a family after they've been working. The other founders may feel like they contributed the unspoken sacrifice of not having kids so subsidizing another person's family at a critical time can seem unfair.