^“True Blood” vampire lesbian EVAN RACHEL WOOD – who’s admitted in press interviews that she’s a lady-lovin’ bisexual – just started flashing an engagement ring after striking a weird bargain with Brit actor/boyfriend JAMIE BELL: She’ll NEVER cheat on him with men, but girls are fair game!

Explained a friend: “Evan says Jamie’s as open-minded as she is, so if she wants to pursue a girl she fancies, he’ll have no problem.”

Wood and Bell are back together after a five-year break – during which Evan was engaged to shock rocker MARILYN MANSON – but pals are saying don’t hold your breath on an actual wedding.

“Evan’s a smart girl and talented actress, but she’s as confused and flighty as they come in romance,” said one.

“I don’t know if their idea of an open marriage is workable.”