I think this is a Harlequin book: She is working for a security company when she finds out that at a company party that her salesman boyfriend has also been wooing and is announcing his engagement to the boss' daughter. She has a one night stand with the boss' son, and then finds out that she is pregnant. She quits the company for awhile, but the boss finds out and tells his son, who finds her and offers her a marriage of convenience to be dissolved later. They search for and purchase a house. Of course, they discover later that they really do love each other. If anyone recognizes this plot, please let me know
