It's already been said several times, but public sex.
I fucking LOVE to watch videos of people fucking out in the open where others can see them, and it's super hot to fantasize about, but I have absolutely no desire to be registered as a sex offender. No matter how good the sex is, it's not worth that risk.
Also I've got pretty crippling BDD so being in any kind of sexual situation where others could see me – even without risk of legal repurcussions -- would just result in a panic attack or something. It wouldn't be great.