Years ago when we still had polls, we conducted a survey among the GGs to ask what bothered them most about the CDing. Was it general turn-off over the idea of their husbands expressing femininity? Fear of transition? Fear of ridicule from strangers or of being ostracized by family and friends?
Fear of ridicule and ostracism won out and this is not inconsistent with the reasons it takes CDs/TGs/MPs (new acronym ... "middlepathers" :p) so long to go out in the mainstream as well. The difference though, is that TGs receive the personal benefit of experiencing the freedom of self-expression, which overrides any negative outcomes such as stares or smirks. The wives don't have this particular benefit, even when they know that going out makes their husbands happy.
So basically, until a wife develops a thicker skin, she feels embarrassed if her husband doesn't pass. She's afraid of being judged, of having people say things behind their backs. If we lived in a world that accepted gender non-conformity this would not be an issue.
My own SO passes/blends well to most people, unless of course we're interacting with someone at length and then they do know that she's not a GG. But I've gotten around the fear that she will be read & judged by telling myself (whether it is true or not) that if anything, people just are expressing a mild curiosity if and when they stare at us and read her. Still, neither of us is ready to be out to the people in our own backyard (who know or chance to know my SO in guy mode) other than our friends who are open to the LGBT community. We live in a small town.