My mother found out years ago, when I was with my first wife. She, being an evangelical, objected highly. I explained why we had an open marriage and that I wouldn't entertain any more complaints from her on the matter. She's not mentioned it since. I've no idea if she noticed that I was dating my recent ex, despite knowing that I was in town visiting and didn't stay at her house, but at the ex's place.
So, while my experience with telling my mother wasn't good, it wasn't as nasty as I think it could have been. I was pleasantly surprised that she didn't try to push the issue after I explained why it was the way it was. She didn't approve, yet she didn't fuss about it after that.