It's a couples retreat because Marge doesn't feel Homer gives her the attention/respect she deserves, so this retreat is time alone (together) with other couples to improve their communication.
Homer however just wants to go fishing, which isn't the purpose of the retreat (and further ignores Marge).
They go to Catfish Lake to work on their marriage and Marge forbids Homer from fishing for some reason.
It's a couples retreat because Marge doesn't feel Homer gives her the attention/respect she deserves, so this retreat is time alone (together) with other couples to improve their communication.
Homer however just wants to go fishing, which isn't the purpose of the retreat (and further ignores Marge).