As someone who is currently going thru this, i say please tell.
I'm finding out about an affair after the fact.. i never would have known if she hadn't told me. Ya gotta respect that no matter what.
But every case is different i guess, i mean we have 20+ years, house, kids, etc. All great years except the last 1. She's so remorseful she's almost physically sick, and can't believe what she did. Right now the cheater is hurting more than the partner. But it still sucks.
Then you have the bf/gf, dating issues. Or a 5 year marriage where someone had a 2 minute kiss after 6 beer and 8 shooters. While it may not be good, maybe the damage on a minor mistake can be kept quiet if it is a one time ever mistake, never to happen again. BUT, if you sleep with someone, to me it isn't minor.
The kiss issue may make more damage being revealed, but the sleeping issue should be addressed to find out the real underlying problem.
But if you want an honest relationship... maybe just stay out of potential trouble situations in the first place.
--Rum