This Rogue of Mine by Amanda Mariel

Lady Cordelia Daventry may be a spinster, but she has not given up all hope. When she finds herself alone with the well-known rogue, Nathaniel Lowell, Viscount Wolverton, she embraces her chance to experience desire. Finding herself in a delicate condition, she turns to Lady Celia Kendal for help. The last thing she expects Celia to do is share her secret with Nathaniel.

Nathaniel may be a rogue, but he is not without honor. When he discovers what he has done, he sets out to make an honorable woman of Cordelia. The last thing he expects is for her to run. Still, he is resolved to marry her at any cost—and marry her, he will.

Cordelia and Nathaniel cannot escape the consequences of their passion, but neither can they agree on a path forward. They engage in a battle of will and wit as they discover what truly matters. But knowing and having are two different things.