As one half of a married couple that plays together I do understand and empathize with the "No couples" advice. In my husband I have a built-in friend and we almost always play complimentary roles so we can fill out a team easily. I feel hesitant to join groups if my husband can't go as well, so it can be hard to just get to know one of us since we're usually joined at the hip in that regard.

There are some aspects where couples/groups can cause drama. For example if you have a static group and one member of the couple is doing poorly, it might cause an issue to sit/replace one member of the couple. Loot can be a problem too for the same reason. A malicious duo could try to manipulate things in their interest due to them being a group. This isn't limited to couples, a clique could have the same effect.

I would say you need to set clear guidelines, then vet and monitor couples/groups in your FC but don't rule them out entirely, you might miss some really valuable additions.