Well, biologically a penis is just an "outie" vagina, since genitals start to develop before the Y Chromosome kicks in.
Edit: Seeing as how this is by far my most downvoted comment, I'd like to clarify that no, I do not believe a penis and a vagina are the same, just flipped back and forth. I'm just arguing that a penis was a vagina that was biologically capable of producing eggs, it just didn't, before it became a penis. Maybe if you have the surgery that changes your genitalia to the gender you fit in with, they are just going through or reversing that process. Seems reasonable, at least to me, seeing as how I'm not a genitalia doctor, or a doctor at all.
TL;DR:I'm not an idiot. I know how genitals work. I was just trying to make a point.