Hi Veight,
If I follow your post you and your GF where just having some sex games and you are both a bit unsure? Please correct me if I'm wrong

. Many couples have the same issues. Many cis guys wear sexy female clothing with there equally cis-girlfriends/wives. It's perfectly normal. It's fantasy stuff. You may have had a fantasy of being penetrated by your GF, it's no big deal, it doesn't mean you are gay or TG. If you look up Pegging on Wiki you will find that anal sex for guys with there girls isn't unusual. But if either of you don't want to go there don't.
You can both explore some of these issues further if you want a bit of FemDom but not going "to far" with stuff like painting/polishing her toe nails. Plaiting her hair. It's fun and sexy for both of you. One that is fun is shampooing and styling each others pubes. Just make sure the hair dryer isn't too hot. And be very careful were each of you shave

If you find you are getting more and more feminine desires then maybe that is time to talk to someone. Otherwise enjoy each other and be open about it.
It's Saturday night in Australia and I bet I could not count how many perfectly heterosexual men are wearing garter belts and stockings. Nothing wrong with that either.
Cindy