It makes perfect sense to me.
True love is the magical mechanism that establishes the protection, just like casting a ward. And like a ward it remains until broken or taken down.
Sex, voluntary or involuntary, with someone other than your True Love breaks the ward. Voluntary is you taking it down, involuntary is an attack that breaches it.
Having sex with your True Love at a later time rebuilds the ward. I imagine Thomas and Justine had to refrain from body contact very shorty after completing the act as the ward came back up.