Why so much anti-May?
I'm not exactly sure why people dislike May so much, except that she "prevents" Newland from running off with Ellen. Not that it would have lasted, because the moment Ellen heard that May was pregnant, she would have kicked Archer back across the Atlantic. He would have been in exactly the same position as before, except disgraced.
As far as I can see, May's only sin was to tell Ellen a half-truth (which turned out to be quite true), and to play the pretty dim little woman as all women of the time were expected to. It's not WRONG to expect your spouse to stick to the commitments they made to you, and to expect them to not run out on you and your child. Nor is it wrong to try to protect your marriage even if it makes somebody unhappy.
Okay, so it made Newland unhappy. He should have thought of that before he married May and got her pregnant. He made HIMSELF unhappy. It's not May's fault that the idiot married and had sex with her, when he was in love with someone else, and LIED TO HER about not loving anyone but her.
Her only overwhelming flaw is that she fell for a sad, pitiful little man who had neither the strength to defy convention, nor the integrity to stick to his commitments. And she stuck with him despite his continuous emotional infidelity and him ignoring her, which shows that she honored those commitments a lot more than he did, despite all his lies and deceptions.