You consent when you download the content. You are not obligated to use Youtube, but if you want to watch funny cat videos for free there you have done so with

You consent when you download the content. You are not obligated to use Youtube, but if you want to watch funny cat videos for free there you have done so with the understanding you'll be shown ads. This is as silly as "taxation is theft", you consent to being taxed when you participate in society(whether that be buying goods, or being paid for producing goods).