Tell her that you're separated.

You don't want to scare her off, she's really cute and seems like she'd definitely be down for some fun. Dumping a story about having an "open marriage" might not be the best thing.
"I'm actually separated. I still wear the ring out of...habit, I guess?" You thumb your wedding ring, looking at it. "My wife and I have been living apart for about 6 months now." You look back at her. "I... sometimes I still hope we can reconcile, but she doesn't seem to want to make the effort."
"I'm sorry... do you mind if I ask what happened?"
"Oh, it's a common story. Married for a while, things get stale, you stop having sex, she starts having sex with someone else..."
"Ouch." her face is full of sympathy. "That sucks."
"It's okay... that's why I'm out tonight, I guess. Trying to get back on the horse. Guess I should take the ring off, huh?"
"No, I understand." she rubs her arms. "Hey, would you mind walking me to my car? I don't think Bradley's going to cause me any trouble, but..."
"Say no more, m'lady." you put on a formal tone. Bella laughs.