Pansexual very obviously =/= gay. If you really don't know what it is, here's a hint: Pan means 'everything' or 'all'.
As for whether Kirk can legitimately be described that way based on canon? The argument isn't nearly as clear cut as you appear to think, since a huge number of his conquests were clearly alien, which begs the question of whether they really all had standard genitals. Also begs the question of whether it would really bother him if they didn't, since he never seems to put in any effort to find out ahead of time. Not to mention some rather obvious sexual possibilities for the prehensile tails on the cat girls in ST09...
Kirk never did the horizontal mambo with anyone who, given the limitations of mid 1960's makeup and budgetry contstraints, wasn't clearly female. The in the original movies, only Carol Marcus was revealed as one of his previous flames in
TWOK, with Catherine Hicks (Gillian, Whale woman) in
The Voyage Home, and a passing reference to an Antonia in the Nexus during
Generations.
I'm not sure the standard or otherwise nature of the "genitalia" on any of those green painted women in TOS is important. As far as Kirk was concerned, he was having heterosexual nookie, if indeed he had any at all. To say otherwise is just seeing things that aren't there, were never hinted at, and that just gives ammunition to people like Pegg.
In what should be a celebration of Star Trek's 50th anniversary, I find what Pegg has done and said, and all the consequences of his actions due to the way he went about doing what he did, to be pathetic, self serving, idiotic, and utterly crass. None of us should have to be here debating and arguing about this because it should've never have been allowed to happen in the first place. Pegg is someone who clearly needs to be kept on a very tight leash, and part of me blames Paramount for not keeping him under control. Justin Lin is also squarely to blame.