I think, if you're really unsure of how she'll feel about it, then it'd be good for you to tell her just that - that if she's not into it, no problem, you won't die without it, nor run to the arms of another woman. I know I'd be a lot more open-minded about trying something if I knew it wasn't a "do it or else" kind of thing. Myself, I think seeing a guy dressed in any kind of girly/femme clothing is hot. My guy feels very sexy when he wears my panties, and I'm more than happy to let him wear them, 'cause Daaaaamn is it hot.