It's been awhile since I watched it, but here's my take.
As noted, Joe was a misfit. This is the wrong decade and wrong location for someone to be unsure about his sexuality. He's trying to assert himself, like being one of the Maguire Sisters...maybe he's just trying it out.
I think if you look at old TV material you can find an interesting convention. Paul Lynde was gay for instance and you see him really pushing some boundaries in his material. An audience today would probably say, "Gee, we GET it, ok? You're gay!" But in those days it was eccentric, silly, maybe campy, something else.
"Gay? Like the Flintstones, 'We'll have a gay old time'? Sure he seems happy to me!"
If you didn't like Paul Lynde, you switched off the set. But Joe is right there in their little community.
I suspect there aren't many universal truths about the gay community...I had a sexuality course once and I'm thinking the teacher said they break down into four or five groups.
1) Knew it all along from an early age, never doubted they were attracted to same-sex.
2) Don't have a big hangup about it, experimented a little.
3) Don't prefer it but they're in jail or at sea in the Navy or an all-female university otherwise have no members of the opposite sex around, so it's gay sex or no sex. Look up "Lesbian until graduation (LUG)," for example.
4) Connect more emotionally with the same sex but the actual acts are secondary. Like "Kissing Jessica Stein" but with true non-sexual passion, if that makes sense.
5) I don't remember.
When the incident happened about Joe getting beat up, I thought, 'Homophobes. Sometimes they're lashing out because they see something in another that is a reflection of themselves. They can't tolerate that part of themselves so they beat him.' If Sissy's husband had sex with him, maybe that was the confirmation that he (the husband) had been attracted to Joe all along.
So let's go back to Jimmy Dean's conception. I think that for some, attraction doesn't have a gender. I can believe that Joe was very sensitive, that he saw beauty in Mona, -and/or- maybe he envied her because she was female, perhaps the female he'd like to be.
It's also possible that he thought if he tried sex with a woman, he could "overcome" his other desires. I don't like that last theory much, though. Reason: when it's all said and done, Jo (Karen Black) doesn't stop loving Mona. I think she's genuinely hurt that Mona is still stuck on James Dean and will never love her like that.
My conclusion then is that what Joe felt for Mona was a transcendent thing. It isn't about sex. Like I said, maybe some people know all along that they're homosexual. So why can't Joe be someone who feels like a man trapped in a woman's body who is also in love with a woman? That's what I think.
Loved "Kissing Jessica Stein." What I got from that movie and your insightful post is that labels cannot explain everything. Categorization is helpful, maybe even essential, but ultimately, solely relying on them is missing out on life's rich complexities.
Glad you found it helpful.
Maybe #5 was bisexuality, i.e. both straight AND gay, equally attracted. It could be that Joe is 99% gay but the 1% that isn't adores Mona.
People are attracted to a variety of things of course and some traits aren't physical.
I bet some "99% gay" men would sleep with Cher in a heartbeat, not because of how she looks or how much fame she has, but because of what they see in her personality. People often seek others who have something they themselves lack and in this case, the link would indicate they see courage, standing up for what's right, and so on.
Life's rich complexity, amen...but very difficult to distill that in 90-120 minutes of film.