Pretty sure it’s that they were a lesbian couple but one realises they’re a guy and the other realises they want kids so they break up but remain besties and try to set the other up

The link has extra panels and a comment from the artists that explains it more. This isn’t a joke so much as it is explaining their situation in a joking manner.

Amicable divorces are apparently funny?

I don’t know. It’s an odd one.

The joke is the absurd and funny statement, “ladies, my wife is single (and you should date her (implied))”. Basically they are best friends who broke up and now they are supporting the other dating by joking absurdity of the situation.

Comics don’t have to have a joke, they can just tell a story. This is a nice story of two people who care for and support each other in spite of discovering their selves and life goals don’t sync with continuing the original relationship. They manage to break up the romance with each feeling freed rather than dumped, so they can continue to be friends. Hopefully their eventual new partners will appreciate this.

Part of the humor comes from subverting the expectations. You might expect this kind of conflict- that one wants kids the other doesn’t, that one wants to change their gender expression into something the other doesn’t like- to cause conflict. Fighting. Anger.

Instead they just fully support each other.

Further, it subverts the mainstream possessiveness of partners. It’s very typical for people to be like “don’t be interested in my partner!” Or to be very uncomfortable with their ex seeing other people. Instead, this person is being very supportive of their person.

A lot of behavior in typical monogamous relationships is really shitty and selfish.

It’s not the funniest thing ever, but that’s how I see the mechanics of it working. Subverting some relationship expectations.