When the sex is so unbelievably awesome that after orgasm you immediately forget what you were talking or thinking about.
I was talking dirty to my wife while we were having sex. She got so excited, clamped down tight and gave me the best orgasm ever. She had to repeat what I had told her since I couldn't remember. Instant case of sexnesia.
The term for the amount of penetration one is allowed to have during sex when others are present.
I'm so damn horny, but since my parents are in the living room we'll have to settle for the angry inch. It's better than nothing!