Definitely hookups, but also not just hookups; I've found gay Tinder to be a really great way to meet people in my area from all walks of life and parts of town who have overlapping interests with me.
I definitely agree that if your service depends on a subscription model and is explicitly aimed at people whose goal begins and ends with finding a life partner and deleting their account, then there is really no easy way to reconcile this.
Hookup apps. Apps for non-monogamous people. You're looking for repeat customers.
The only other way is paying when the couple gets into something long-term, but the fee would probably be higher than people want.