God damn man, what a sad story.
Let's ignore the legality issues (fucking someone while being well aware you're positive), what about some type of moral dilemma. Is this girl, that heartless? I know shit is rough, and troublesome to cope with. But an eye for an eye mentality. "Hey I got it, now everyone has to suffer" is really really fucked.
BVF - End it now, and put a stake through that bitches heart. If that doesn't work, try some garlic and holy water.