... he couldn't touch her unless she wore a rubber suit to protect him from being burned.. It wasn't until she had sex with another that that changed and then she could have sex once more with Thomas.. However if it was always that simple, why didn't she do it sooner? ...
This bothers me, too.
Not just with the Thomas/Justine: why TF is this (obvious) method not part of the toolkit that every whampire knows??!?
They use sex for feeding their Hunger... but
also as a critical tool for political power.
They're used to thinking of sex, and sexual permutations, as tools.
They have zero expectations of sexual fidelity: each Whampire has multiple kine, and many kine get passed around amongst the Whamps; gifted, loaned, traded, &c.
They know of "sex outside of love" as the "magic key" to get past the "True Love" protections.
They
should know "have willing, friendly sex to break TL protections" and then renew said protections (when they make love to their TL) as the way to move forward.