A variant on the word "fluctuation", commonly used in the world of economics to identify irregular/varying market movements. However, this term is used specifically in reference to sexual targets of one's liking. In other words, "fucktuation," defines the irregular change in taste of men and/or women when put into a sexual connotation.
1) "Yea, seems like Clinton has taken a liking towards younger girls now."
"I dunno man...he used to date older women, maybe he's going through a fucktuation."
2) "Yo bro, should I go for the younger or older chick tonight? I can't decide dude..."
" Make up your fucking mind and stop fucking fucktuating."