A Gay mate of mine once said something that has always stuck with me and could be very true for a lot of people "Depending how much you've drunk, you are only one drink away from doing something Gay"
lol, in general my take on being gay or not would come down to, choice, nothing more or less.
When you look at straight people alone, you can ask 10 different men and find that, shown 20 different women, each man has an individual taste, finding some of the women repulsive, some ok, some stunning. So what can we conclude, each one is born with different gene's which determine what they find attractive?
What about anyone and art, one person can find a painting fantastic and deeply moving, another person might look at it and think its half a dead sheep and not art in the slightest.
Why do we not question why man A loves a big boobed, tall blonde, while man B loves a ginger midget with a huge ass and no boobs, but we have to find some other reason that man C finds another man attractive?
Simple fact is, people grow up with different tastes in EVERYTHING, including what kind of women you like, why on earth is anyone surprised that this extends to if you like men as well as or instead of women, what else could possibly need explaining?
Is it that more people are gay now than ever before, possibly, with the amount people have hidden it throughout the millenia its literally impossible to know if a higher percentage of people are gay now than 500 years ago.
Would that have anything to do with instinct, possibly. Most animals instinct is to procreate and strengthen the pack, however the longer we live without "needing" to increase the population the less that instinct might drive us, that doesn't mean people weren't attracted to men before, just that they felt or realised the need to procreate as more important.
Can the guy in questions tastes have changed based on a stroke, can't see why not. Medical conditions have caused all kinds of changes in people before, could he have always been gay, sure, is there a chance the stroke did it, sure, is there a chance Stoke did it, sure.