The problems with some startups is that the founders often do not have any experience at all about how to manage a company with employees. So you can not expect much professionalism from them. If you want to work professionally (and you do - an unprofessional business partner is very likely to take your work without paying), you need to take the lead and teach them what "professional" means.
A good way to show them they are playing in the grown-ups league now is dropping the word "contract". Come up with your own contract if necessary (feel free to be bold in the fineprint) and insist that you won't communicate with them about any other topic except the contract until they signed it.
Yes it is likely that this might scare them off. But that's a good thing. Anyone scared off by being told to commit to a contract isn't serious about their project anyway. You should better avoid making business with such people, unless you are OK with investing time and effort into something which will never get you a cent of payment.
For more information about why contracts are so important when freelancing I recommend this presentation by Mike Monteiro: "F*ck you. Pay me."