Before Star, who was the poster girl for female Cuckery?

It wasn’t a very good episode anyway.

Good question

you've mistaken childhood puppy love for true romance?

It's the one straw Finngum shippers have to grasp. She kissed him on a cheek as a kid so of course she was totally DTF as an adult. Finn was cucked so hard, it's not even funny. Both romantically and socially.

How exactly does that episode disprove that it wasn't just puppy love?

It wasn't any kind of love. Finn was just infatuated with her because she's a princess and PB just used him as free manpower.

Just like 616 Gwen

why does she fight the forces of evil?

Because she needs something to do while Marco is sleeping with other women.