You talk as though crossdressing is somehow a less valid reason for buying something. Why is wanting a dress for the prom a "better" reason than a guy wanting it so he can feel beautiful? And when you think about it... if a GG were to buy the dress purely for prom it's likely to only be worn once, whereas in your case I'm sure it will be worn again and again and possibly cherished more for it. Either way, the dress ends up with someone who will make use of it and love it so... why feel guilty?