Hey, no replies yet, so maybe this'll inspire others to contribute.
Outer - and it was quite a while before I tried inner. Years, even.
Condensed backstory is the first time I tried on clothes that weren't conventionally worn by my sex was as a kid when we swapped clothes around to see what we looked like in them and what it felt like to wear clothes of another sex. So it was a mutual thing and most of the interest was on the girls' side, but I remember feeling that something deep inside was more comfortable with this than the rest of me said it should be. It was definitely an inflection point, not that I realised it at the time, but I couldn't ever quite get it out of my mind.