Lara will "encourage" whatever she wants, but Harry is a stubborn man and he is still inlove of Murphy. Do you think he will agree to sleep with anyone else, just to please Lara? As I said in other place, yes, Mab herself can force him to break the protection, but if she does that, Harry will be a sort of hostile hostage, a much less useful WK. I don't think any of them risks force Harry to do something like that.
And, as I said before, I think the whole reason for the sex with Murphy is giving Harry the protection. JB won't write it only to have a moment of Lara's bow burning while rescuing Thomas, so protections has an important part to play in the future. Perhaps even to protect Harry of other wampires.