It turns out that one of a Guardian's formal duties is also to comfort & satisfy a Summoner sexually

(one of the reasons why Dona sneers at Yuna for having so many guardians)

Tidus, as usual, is completely oblivious to this (despite the other party members hinting at it) until he's suddenly ushered into some weird sex ritual with Yuna

Optional: Yuna x any of her other guardians, too