Because while it was a rape, and while she did feel completely violated I'm pretty sure that Joan doesn't consider it a rape. Legally back then it was considered impossible for a husband to rape his wife. And I think she's already extended that mindset to her fiance.
But I also think that all of the events of Joan's arc this season will serve to radicalize her later on. But not this year.
Yesyesyes. Horribly, regretfully yes. Joan's entire arc this entire season has been about how constricting, how crushing the framework is within which she operates--and how totally unable she is to really see it, or imagine herself outside of it.
She's made a name for herself, and a huge central role in the universe of Sterling Cooper, as an übercompetent Queen Bee of the office, effortless manager/diplomat/manipulator of the office environment (the steno pool interacting with the bosses, the creatives vs. the numbers guys, the partners vs. everyone else, everyone in the company with the clients--when her cattiness doesn't get away from her she's just masterful at juggling all these incredibly complex relationships).
But all that incredible competence and deftness go on under a heavy glaze of Marilyn, the only real power symbol available to a young woman of the time. Joan's ferociously bright, ferociously good at her job, but she doesn't recognize her own real value. She does her best to crush Jane like a bug not because Jane has any office or business smarts at all, but because Jane is lovely and willowy and has Joan's own former boy-harem all in a twitter.
That post-firing scene where Joan bears down on Jane like a magnificent battleship about to crush a rowboat? Joan was terrifying and powerful, but the difference in costuming was acute: that scene was the climax of the costumers' work all this season to make her look more and more architectural--rigging and struts and flying buttresses making her look more and more massive, and stiff, and inhuman--as Jane sat there, a birdlike little Audrey Hepburn in a thin little top covering a slender little figure that had no support and needed none. Total character shorthand through costuming: you saw that, you knew Jane might be crushed, but you could also see Joan getting more and more trapped.
More vast, more rigid, closer to faintly ridiculous year after year after year, and even if she crushed this Jane there'd be another Jane the next year, smaller and nimbler and younger still. And another, and another. That battleground, that value that Joan has placed on herself, can be won but it can't be held. Eventually she'll fall.
And she knows it, so of course marriage has been her ultimate goal all along; and, at past thirty, there's no way she won't choke down her own violation and smile and say yes anyway.
Brilliant office manager. Brilliant at dealing with clients. Brilliant at script reading and analysis and brainstorming. And she doesn't even see it, really. Her fiance rolls his eyes at her enthusiasm, and she smiles politely and takes it; someone's promoted right past her and she gets a pat on the head for all her work, and she's bothered but doesn't let herself really think about why. Her sexual advances are brusquely refused by the man she loves, and she sucks it up. And she'll suck this up too, because nothing else about her counts, not even to herself. She's past thirty; if she sends this guy away, she may never get another chance to marry, and if she never marries she has no value at all.
I hopehopehope she gets radicalized, but with one episode to go? It won't be this year.