Yeah, I always figured Kirk sees flirting as another tool in his arsenal - if he can use it to help his ship and his crew, why wouldn't he? And given the number of times Kirk flirting with/pretending to seduce some alien woman saved all their lives, Spock would be kind of stupid to try and stop it. He knows Kirk would never actually cheat on him, so what's wrong with a little flirting?

Anyway, glad you liked the fic!