Two people (probably - it is all probabilistic) wouldn't be able to repopulate the earth anyway. If I recall correctly the minimum number of people to likely
do this would be something around 30 or 40 (although it depends greatly on the genetic diversity of the people involved and how many children they would have), and the lowest the human population ever got was after a super-volcano exploded and the estimate was a few dozen to hundreds of people left alive after that event. Although I don't know the details of how all of that was calculated/hypothesized.
But to answer the gist of your question, if she were attractive enough I'd probably have sex with her, since all other options are gone, and I'm a horn dog.
And we'd be pretty dependent on one another for the human connection, anyway. I'd probably be against having children though. Wouldn't want to put the burden of being the only people alive on another generation. I'd let humans go extinct.
I don't know if this makes me bi or not, but I don't find vaginas (or rather women) inherently undo-able, especially if that was all that was left. I just prefer guys overwhelmingly more. I think if she were unattractive that would be a bigger deal breaker than her gender, to be honest. For reference, I consider myself gay, and have never had sex with a girl.