Seeing that her twin brother-now-sister Max (who now went by the name Maxine, apparently) was fully engaged in kissing Dustin, Nikki (who knew that she was supposed to be a boy named Ricky) saw her opportunity to steal the Cronivac device.
Once she had it, she was going to use it to change herself back into Ricky, but paused when she watched the intense make-out session between Maxine and Dustin. She felt that Max deserved to stay female; at least for a little while. After all, Max was the one who (though, unintentionally) caused Ricky to become Nikki. But instead of changing her back to normal, he just kept her like this, even making her think she was supposed to be a girl! Even if it was for a short time. So Max needed to stay female. But he/she shouldn't have the luxury of getting the boy (Dustin) either.
So that's why she decided to take him off the board completely, in a way. Making Dustin the target of the Cronivac, she changed him from male to female, but kept sexual orientation the same. This new girl, whose name became Destiny, was still straight, so she was only attracted to guys. She stopped kissing Maxine almost immediately, as it felt all so wrong. But what was her awareness? Did she think she was supposed to be female, or did she still remember being Dustin?