Down for the Count (Dare Me Book 1) by Christine Bell

Truth or dare...

When Lacey Garrity finds her groom in flagrante delicto in the reception hall closet with her bridesmaid, she's saved by her best friend's older brother--childhood tormentor, crush, and boxing bad boy Galen Thomas. Galen's solution to her problem is both exciting and dangerous. What better way to forget the mess of her life than go on her honeymoon with a hot guy who can't promise anything beyond today?

...or TKO?

Galen had been counting on Lacey's wedding to put her out of reach--and out of his mind--once and for all, but their steamy Puerto Rican escape is testing all his boundaries. Now that Lacey's embracing her inner bad girl, Galen is tempted to throw in the towel and claim her for himself. But with the biggest fight of his career on the line and an important business merger threatening to derail Lacey's resolve, their romance might be down for the count before it even begins.