That comparison surprised me since Peggy isn't somebody I'd describe as desperate for a man anyway. Joan, to be more specific, was desperate to be married more so than a man. She could have lots of men, but she was getting to the point where by her own rules she was an old maid.
She wasn't pumping life support into her marriage at that point. However clearly a bad choice Greg was, at the point Joan got pregnant her marriage was not on life support. She was being a regular wife and the two of them were trying to have a child, which Greg also wanted.
As for giving tacit approval to the rape yes, of course she did--as she was probably raised to do and most people on the show probably would have done as well. Saying he belonged in jail is meaningless because he would never have gone to jail for such a thing no matter what.
No, she wasn't a woman in 2020 who simply wanted a child and doing it alone was an obvious choice but when did she ever pretend she wanted to be some trailblazer who just wanted a child and didn't care what society thought? She wasn't just baby-crazy, she wanted to get married and have a child, just like Betty and Trudy. So she was a married woman having a child with her husband who was going to be away for a limited period of time but then come back and have a normal life with them. It wasn't until after Kevin was born that Greg made yet another decision on his own to go back to Vietnam. Before then Joan had every reason to think her marriage was working fine. Once she saw what it was going to be she pulled the plug pretty decisively.
So why not just take the fact that Joan's husband dumped her as the punishment Joan got? Because I really don't think Greg getting up on a high horse and calling her a hussy and a liar would have made Joan look bad. I'm not sure how else the show was supposed to be addressing this interesting topic of, I guess, women lying about paternity. A flashforward to Kevin doing a 23 and me in 2019 and calling Joan a lying whore? Greg coming back from Vietnam and suddenly being a wonderful father who then, in a soapy twist, turns out to somehow be biologically proved that he can't be Kevin's father and tearfully vows to be Kevin's father anyway while Joan looks ashamed? A different soapy twist where Roger blurts out the truth and Kevin looks wounded in his high chair?
If Greg's being a terrible person protects Joan from the moral implications of lying about Kevin, so does Joan's being a terrible person protect Greg from the moral implications of abandoning his child, obviously. So what's the problem? Neither of them is getting away with anything or getting to be the victim. Both of them can be judged by the audience for exactly what they did. Joan's never shown as a victim in terms of her lie, as if it was something she was forced into by society. On the contrary, it's shown to be completely something she does because it lines up with what she wants.