And if you consider every day of a romantic relationship as just a one-night stand with them (by coincidence the same person as the night before but no matter, that's just a coincidence), then you'll never be hurt if they cheat on you or even outright leave you. You can never be hurt!

To me, saying "always value the equity at zero" is exactly the same as that. Like, "always value the relationship at zero."

It's not sane advice in my personal experience, especially at the tiniest company sizes.