Obviously, being a comedy, Shaw was going for laughs as much as he could get. Misalliance would probably rate a lot higher for laughs (especially about age differences) than does Candida. Because Marchbanks can't appear to be a viable choice for Candida other than any temporary young lover in British society, the character then plays the silly foolish boy for all it's worth, tripping over the furniture, etc. expressing his immaturity, lost in his attraction for the older Candida. Because the Brits are great at concealing the emotions, anyone who lets his emotions wreak havoc thus is to be portrayed as the fool. If Shaw really wanted Candida to have a real choice, he would not have directed so many laughs at Marchbanks, but more for the windbag Morrell, Candida's husband. She doesn't choose Morrell to be "moral," but because it's the thing to do for all good British wives who at least felt they deserved the power of running a household on their own terms. She prefers the comfort of dictating to her "sweet baby James," to the unsure future of running off with a youngster who rejects the standard British notion that containing one's emotions is paramount. Don't forget--Shaw said he didn't "like" Candida after he had written her that way. But he felt Ibsen's A Doll House needed a response, so he went as opposite as he could. A pity.