Better than saying no is using some negotiating tactics to take the pressure off you.
Use an external constraint and pre-disqualify yourself. So one day when you're not being asked out, ask to have a talk with the founders.
"Hey guys, had this talk with my significant other last night and they told me that while it was okay to put in some extra hours, I couldn't be burning both ends of the candle by doing both tech and business work. It makes me impossible to get along with, cranky and miserable."
This sets your limits, puts the onus on some outside party, and presents the founders with a constraint at a time when the constrain is meaningless -- after all, they don't need you now. Then when an actual event comes up, gently remind them that you've already had this conversation. Offer to talk sometime during the day. Hell, have the biz folks come by the next standup. That way they can talk to the whole team.
The point is, you're not trying to hurt the founders, you're just placing some reasonable constraints on what's possible and what's not possible. It's not your idea (and in reality, it's not, it's just the way you are), and it's not the end of the world.
At the end of the day, you have to set limits. There are tricks to setting limits that reduce conflict. You should use them. But as long as you're needed for something you'd prefer not to do, you'll never completely eliminate that stress.