It took me a long time to come up with why. For me, it's about two people, Jennifer and Mike. Mike is rough and tough - he builds and races off road trucks, can fix anything, likes being dirty and working with his hands. He's also feminine somewhat - likes to go to the theater, loves to cook, etc.
Jennifer is someone that I can be to honestly express my feminine side. There is so much man going on when Mike's around that my femininity gets bottled up, so to speak. When I am Jennifer I can truly relax. I become more comfortable - I mean really, what is more comfortable a skirt or jeans? I get to be someone I'm not for a period of time. I get to be cute, fun, frilly, sexy, colorful. Crossdressing for me isn't about being straight or gay - I'm straight - it's about releasing the inner femininity that is inside me. Jennifer is an extension of who I am, without her I am only partially me.
Women (GG) can express masculinity whenever they want - jeans and a t-shirt and hair tucked into a baseball cap is just fine. A man tries to let out a little femininity and the world goes nuts. Understand that your boyfriend is only expressing a part of him - and it's a part of him that makes him the guy you love. A macho dude that represses all his femininity is not someone that is as sensative to the world as someone who is honest with themselves about who they are.
My experience is that CD's are some of the greatest guys AND girls you'd ever want to meet. We're aware of our feelings. Aware of others feelings. We can laugh with you when you say your hose are creeping south and that your heels are killing you.
Men are supposed to be men. And Mike is. Jennifer is a part of Mike - the part that makes him a sensative and caring person and lover. Being in love with a CD is a bonus, not a minus. Instead of repressing who he is, he's expressing it - and that is not something that a "normal" man has the balls to do.