Okay so it looks like your best option is
The Robin Romance Mod, which is theoretically compatible with
Free Love.
It replaces Robin with her sister Veronica as Sebastian and Maru's mother/the town carpenter/Demetrius's wife, but that can be disabled and leave you with two Robins that never acknowledge each other, one who is single and lives with Leah, and the other who is happily married. I'm pretty sure I've seen her and the other non single characters in other mods (which definitely had Free Love or other polyamorous mod compatibilities) before, though, so I'd advise that you keep looking if this one doesn't work for you! best of luck!