When a Plane Seat Next to a Woman Is Against Orthodox Faith

or you could do the opposite - not sure how he would take it -- say you were once a man so considering that is your original gender then he should not have an issue sitting next to you.

Or tell him you are a lesbian so it is nul and void. I like confusing people.

I saw a movie once that as a man convinced me I am a Lesbian .

Depending on who you ask, and how honestly they answer, the reasoning is either "because we respect women and don't wish to touch them in the manner a husband might," (the bs version heh,) or if honest, "touching a menstruating women makes one unclean." And since the way one gets re-clean is a bit involved it's easier to just not touch any women.

But it's entirely religious, don't take it personally. It's not sexism in the secular sense.