Once Upon a Lady by Addie Jo Ryleigh

Respectable Lady Katherine Baxton, striving to meet the requirements of her station, has become the ultimate dutiful daughter. And now, the Duke of Blackthorn’s betrothed. Far from a love match, Kate is nevertheless determined to do as expected and marry.
That all changes the night she panics at her impending future and runs, stumbling upon a private grove, a mysterious tree . . . and a half-naked man.

The youngest son of a viscount widely thought to have purchased his title, Jackson Cooper demonstrates his disdain for the aristocracy by affording himself every luxury available—drinking, wenching, and gambling—while eschewing anything representing the ton. Jackson has little care for his reputation and no desire to marry. His escape from London is all but complete.
Until fate—in the form of a beautiful, mysterious lady—interrupts his plans, enticing him with the very thing he never wanted.