My theory is that Joe is gay and is (was maybe) in love with Nathan. It explains why he was so venomous towards Gale and why he’s held a grudge as he sees her as taking Nathan away from him. I don’t know if anything ever actually happened between Nathan and Joe but maybe it did and that explains why Nathan cut him out completely and why Joe always seems like there’s more he wants to say, and more of a reaction that he’s after. I also think that’s why he keeps going on about Nathan having a baby.
In terms of why I think Joe is gay outside of the Nathan drama specifically, he forms these incredibly close, intense relationships with men while shagging any woman who’ll have him (methinks he doth protest too much etc!) while simultaneously treating the women with barely concealed contempt, like he thinks they’re stupid for not seeing the truth. He foists himself into the middle of relationships because he wants the man to pick him, he did it with both Nathan and cooper (cooper is another
hole but whyyyy would Joe gleefully accept an invitation to join cooper and kat on what should’ve been a romantic few days?!)
I may be way off the mark, but that’s my theory and I am sticking to it

None of this is to defend Nathan in any way, of course. The guys a piece of
tit!