The Joan thing is real-world implausible, but within the universe of the show, I think it's consistent with the way they position her as a woman of the '50s and Peggy as a woman of the '60s. Joan, fundamentally, believes that her physical charms are more powerful than her intelligence. Her mistaken belief that Don, on the heels of their recent reconnection, approved of Pete's proposition, simply confirmed her worst doubts.
From her perspective, her prospects must look pretty dismal. She already believes that no man she'd want would want her with child attached; she clearly despises her mother but can't do without her; she craves the respectability of being a married woman so much that she married her rapist; and her career has a glass ceiling three inches thick. It's a way out and up. Plus, Sterling, Draper, & Campbell would all shag a female client if it would get the business!