It is okay for people to do whatever they damn well like in their relationships, whether married or not, for god's sake, what the hell has it got to do with anyone else whether the wife/husband is shagging the milkman/having it off with the local rugby team etc etc.
What goes on between consenting adults in private is entirely up to them, period.
I would never tell anyone what to do or whether something was "okay".
Just have a look at all the dating agencies for people in relationships - obviously there is a demand. I am not saying whether it is okay or not. I don't really care one way or another, to be honest, I am not that interested in other peoples's sex lives
Really, I mean it. If my other half wanted to have other relationships, he can, I would never stand in his way. Fact is, he can't be bothered and neither can I - but I can understand why people do, and that is up to them.
People deceive each other all the time in all sorts of ways - infidelity is at one extreme of the spectrum but what about all those "good" deceits eg: not mentioning when the other person has put on weight/you do not find them so attractive/you used to fancy their best friend etc, etc. There are all sorts of reasons why people reveal some information and conceal other information.