Not consenting to sit beside a woman however is CLEARLY discriminatory.

Other behavior aside, how?
Following Merriam Webster:

Discrimination: the act, practice, or an instance of unfairly treating a person or group differently from other people or groups on a class or categorical basis (such as race, religion, gender, or sexual orientation)

Is this even treating someone else like anything other than yourself in the first place?*
Sitting next to someone wouldn't provide anything. In fact, it's quite negative.
* Well, perhaps in this case that means unfairly treating people of your own gender by increasing possibility of someone sitting next to them in certain scenarios, which would fullfil the definition of discrimination.

In this case, if there were no other seats available, simply not taking the flight would have caused no harm. Actually, it would have made one passenger who'd keep more space happy. While it could be argued the passenger got better experience unfairly, based on her gender, I doubt anyone would give that a serious thought.

(My comment concerns someone voluntarily separating themselves)