The love between Thomas and Justine is true love, that is why he could not touch her until she had physical sex with someone else.. Remember she invited a gay woman friend at the end of Ghost Story? Since though she might be mildly bi-sexual, she isn't gay, so the gay sex counts to break the true love lock that prevented Thomas from touching her and at the same time in their minds at least since she isn't gay stayed faithful.
Love tokens like a wedding ring or a rose can burn if they were exchanged in true love. It's complicated, the best theory I
can think of is the Hunger Demon feeds on carnal lust and the "hormones/emotions" that calls forth, but true love is on a higher level... Kind of like getting too near the sun, the Hunger Demon feeds on the light and warmth until death of the victim. Think of true love itself as the sun, touch it and you burn up.
The weird thing is we never seem to hear about any other love than the romantic kind sufficing. Sure, the soul-soul contact is pretty intense during sex, but so presumably is that during breastfeeding, yet there's no general trend of virgins (whose last extremely intimate contact would often be with someone who loved them) being protected. Note trend, not saying it's a total certainty.
So there seems to be something else going on. Something deeper, more specific, than just intimacy backed by love.
I am running on thin ice, obviously, but an attempt: Romantic love is a kind of ongoing sharing of soul between equals. Where a parent's love might seem to be a way to 'feed' the child's soul without quite the same in reverse, a romantic love is a bi-directional feed. And as the hunger demon attempts to feed on the soul, the soul is not really just there, but also elsewhere (namely with the loved one). The demon tries to munch anyway and as a result is halfway ripped from the body of the vampire,
even when the other lover is close, because it's a metaphysical rather than physical distance. The demon could eat on, but it would be permanently dislocated and the vampire-body will die, leaving the demon untethered and forced to return to the NN, before it can do permanent damage to the lovers (after all, a WCV can feed quite deeply without killing).