I would never be all right with such a proposition. A relationship to me should be between two people only--that means that while we're in the relationship, no sex with anyone else, even if it's "devoid of feelings."
well, looks like the OP bailed after trying to run some damage control and having a few home truths forced down his throat. I'm just sure he and his 20 year old fiancee who's already looking to sleep with other people will live "happily ever after".