For example: men receiving women care product ads, or advertising for baby products when I'm single etc.
Even if it's not related to ads, think recommendation sites for books or movies.
Basically this entire thing will at least for a while push us back in the Web 1.0 times. I guess that eventually someone will come with some way to obtain the same results as today (or even better) while completely skirting this regulation.
Edit: Also as all tech regulations, this one also completely ignores the human factor. Bounce rate will increase until people get accustomed to just clicking 'yes' especially if the 'no' button drawbacks are presented in a loss aversion way (imagine big red letters warning about big problems if the 'yes' button is not clicked)