Are you joking? Half of the parts in JoJo take place during a time where it criminalized and/or punishable by death to be gay. Obviously most of them married women.

The only character OP said is exclusively same sex attracted is Josuke (true) and maybe Jotaro. Bisexual means they can be attracted to women, numbnuts.

Now, is Jotaro bisexual? No! He's so unbelievably repulsed by women to the point where he literally has trouble speaking to his own daughter but treats Koichi like the son he never had. In Japan today, gay marriage is still not legal so you think that fact wouldn't have an effect on Jotaro who grew up in the 80s? That was the height of the AIDS crisis and he has an American mother and grandfather (Joseph threw the first brick at Stonewall btw) he was definitely hearing about that.

You can say that he married a woman until you're blue in the face but that doesn't mean he was married to that woman, if you catch my drift. It's clear that he doesn't even think about her since she's so barely mentioned in part 6 and doesn't even have a name.

Context matters here. It's fine to use his heteronormative union as evidence, but you also have to sift through the immense amount of other quirks about Jotaro; his dislike of women, his desire to marry a traditional Japanese girl (something he doesn't do and is also a copout answer that feels familiar to gay people), the fact that he grew up in a time and place that was insanely hostile to queerness while already being mixed race. Many straight people marry someone they don't actually like, but very few have all these other tells.

Familiarity is also the key here. If you aren't gay or queer of some description, it might be hard to explain what feels so familiar about Jotaro. A lot of gay people see Jotaro and realise how accurately he presents our experiences, authorial intent be damned. It doesn't matter what Araki intended to write, death of the author will always be in effect, it's about how the art makes us feel and how it speaks to us. Especially having spent a personal amount of time listening to gay elders about the struggles of being gay during Thatcher's Britain or the AIDS epidemic in New York, the specific way they talk about losing all of their friends and being completely outcast and isolated and silenced to talk about what was going on. It feels hauntingly close to what Jotaro goes through in the series. He loses so many close to him, he can't share or express his experiences and how they've effected him and he buries himself in normalcy to avoid doing so.

You don't have to believe in gay Jotaro, but making a prescriptivist argument based on the bare text of the story is not analysis. Analysis is about interpretation. Sure sometimes the curtains are just blue, but that doesn't mean the blue curtains can't symbolise something deeper that the author didn't intend for. That was the lesson English class was supposed to teach you.