One thing I haven't seen mentioned is any discussion of what she might want sexually. It sounds like the two of you do have a sexual relationship -- when was the last time you sat down on a comfortable couch together, maybe with a glass of wine to relax you, and had a conversation about what sexual activities each of you would like more of? Most women get used to not getting what they want -- there's some position that works best for the woman, or some toy, or some fantasy they'd like to play out with the guy. But because it doesn't turn on the guy, the woman stops asking for that position, or toy, or fantasy.
I think it might help older couples to stop focusing so much on what generates the erection, and give more attention to the woman's pleasure (and satisfying it in ways that don't require an erection). If you can find things that she really really loves, then she may be appreciative of your efforts and happy to give back to you in the way you want most (that is, by supporting your cross-dressing). But as long as she feels that her own needs as a sexual woman are unimportant to you, she's not going to feel generous toward you.
I could be totally off-base here, but I just wanted to bring it up. In my own marriage, I have certain needs (for BDSM), and I appreciate the times when my husband makes an effort to go there for me. And in turn, that makes me more understanding of his own particular needs.
