True Love's Protection

[Mira]

But only sex with the loved one can seal True Love, right?

 I don't think that's been spelled out, but sex with another can ruin it.  Before he fell in love with her Thomas did have sex with Justine because that's when he fed off of her.  However once there was true love between them it was mentioned more than once, specifically I remember a rubber suit of some sort Justine was wearing so she and Thomas could snuggle without him burning her.

So it's hard to say.

[Dina]

But that is what I meant. It has been clearly established but in order to have True Love protection you need to be very, very, very in love with someone who loves you in return AND have sex with that person. That is when you got the Protection achievement. Then, if you have sex with someone else, willingly or not, that is broken. I imagine if you just stopped loving someone or get in love with someone else, that breaks the protection too.
That is why is so difficult to explain what happened to Harry's protection, he was protected in BG. Then he is not. Murphy died, but that is never given as an explanation for the protection breaking, and Harry clearly did not sex with everyone unless he did it and forgot (not the first time it would happen in the show) or stopped loving Murphy. That is why Mab explained the thing about Harry not feeling worthy and being guilty and all that. And...that is what many of us don't like.