Having sex with someone so horrible you force you mind to block out all memories of the entire preceding 24 hours
Dude...what happened on Thursday, that chick was so ugly I have sexnesia...
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.