I'm not just comfortable being female. I love being female! I find it hard to understand why anyone wouldn't want to be female. I'm very much a girly girl, with mostly stereotypically "male" interests.
Of course, it's entirely possible I'd feel the same about being male, if I were male. I've never been male, so I don't know. I probably wouldn't miss the girly stuff, since it's unlikely I'd have tried it.
I think in terms of brain chemistry, I'm pretty much neutral. It's why I love things like makeup and heels and skirts, but also love football, video games, and action movies. Before we were a couple, my husband said I was his "girl friend who likes cool guy stuff." I'm much more at home being "one of the guys" than "one of the girls." My ex-fiance's roommate once walked up to me after a bad date and stated, "I don't understand your gender! Wait, I forgot, you are a guy in a skirt!" I wasn't a bit offended, as I like being one of the guys just fine.