Wouldn't happen, but if it did, she'd probably chop my nuts off.
And I'd deserve it (regardless of their gender).
It's selfish and it's cruel. There's nothing wrong with not wanting to be in a monogamous relationship, but if that's the case, this is something your partner should be made aware of long before anything happens.
(I should note this is all assuming by 'lover,' you mean partner in a monogamous relationship.)