She's Tricked

There's not exactly a lot of girls at the Land of Departure, combine that with how attractive, strong, and kind Aqua is and it's no wonder that Ventus and Terra both have crushes on her.

After finally being given permission to travel to other worlds and accidentally catching a glimpse at Aqua changing, they couldn't take it anymore. They had to see more of Aqua, to do more lewd things with her... So they decided to trick her.