While this may be true to a degree for some women, it is a very wide generalization to make. I think that most women know that the "knight in shining armor" image is not realistic, especially as they get older and experience relationships with multiple partners. You are discounting the fact that many women are turned off by other women sexually, and this is the bigger reason for a lack of enthusiastic acceptance and support.
Even if in the beginning of the marriage a wife knows that her husband CDs, but she is told or her understanding of it is at best superficial (having had no prior exposure) and she believes that her husband is fundamentally a guy who occasionally dresses in order to get in touch with his softer side, and then as time passes she comes to realize that his femme identity is much deeper, she will have trouble dealing with this. I'm not saying that she can't, just that she is discovering that something fundamental about her husband has changed. The behaviors associated with the CDing increase and also change (CDing is seldom static and for most CDers it does progress and expand), as does the focus and the importance it takes on in the relationship. In time, the wife realizes that the CDing is so much more than just putting on feminine clothing and this will affect how she thinks about her husband.
Even in your example above, you imply that a CDer cannot take on the manly role in a relationship that a woman looks for when she marries. I've read some CDs in this forum say that they don't view their roles in the marriage changing with the CDing, but many other CDs fundamentally and increasingly want to be women too. This shows up in their everyday male appearance (they become more feminized), in the way they want to be seen and treated by others, and even in bed. They prefer to take on a more passive voice in the relationship.
I do agree with you though that it's hard for many wives to come to terms with the negative ways that their family, friends, and society as a whole view the CDing. In principle people are for the most part OK with it, as long as it is not in their own back yards.I think this is also hard for CDs to come to terms with and it is what keeps many in the closet.