Sex again?
The joke is homophobia
Is it tho? I get that the first panel can be read homophobic but the end subverts all gender norms. If anything, nail and screw are the same gender
That is the point. It is depicting two sets of heterosexual relationships that do not work out because the individuals are just not compatible. But instead of the individuals moving onto finding the right heterosexual partner for them, they get the bright idea to just resort to a homosexual relationship. The joke is just “gays are dumb”.
If still not sure if you’re saying this is a homophobic joke or a joke about homophobia. If I understand you correctly, the joke is that the gays are so stupid, they didn’t realize they want a same sex partner until the light ball told them?