I had an 18-month affair with a married man (no kids). I used to ask questions on forums like this as I just wanted someone to tell me that it was acceptable and that he would be mine one day. I can honestly say that in the whole time the affair lasted, i did believe that in time he would leave his wife. Looking back now, I cannot believe how naive i was, although I have to admit, the 'part-time' relationship did kind of suit me at the time.
It ended the weekend I met my boyfriend and we have been together nearly a year now.
Funnily enough, Mr Affair's wife is now pregnant (he did not deny it when I said it was a patch-up baby).
I do not speak to him now and would not be surprised if i heard, in the future, that he has cheated again.
I never met his wife and still would not know her if i passed her in the street.
Anyway, after that long answer, I will answer your actual question and say that yes i believe affairs can be acceptable/excusable but, at the same time, i believe they are dead-end and should always be seen as such.
Do not ever plan a future with Mr/Mrs Affair.