So many options, still:
1. A quint drugs Fuu and shotguns him into marrying her.
2. A quint puts an aphrodisiac into Fuu's food, makes him taste it... and out of guilt Fuu marries her.
3. A quint threatens to eat Raiha if Fuu doesn't choose her, Fuu gives in.
4. A quint hires some professional thugs to hold him down while she gets busy with Fuu, forcing him into marrying her.
5. A quint directly dropkicks Fuu, and after an amnesia arc Fuu marries her.
6. The quints read up on Zetsubou-sensei's ending, decide to do the same.
Welp, whatever happens, I'm getting my popcorn ready.
Other options would be: the imouto route, the old friend route, the crossdressing father route, the BL route...