I really dont think so. At the begininng it is very possible to start off as friends but eventually one of the people will start having feelings for the other. Every show Ive ever watched or any male-female relationship Ive ever been in this was the case.
I dont have never cheated but I suspect most of the times when a male or female cheats on their partner for the first time I belief it is with a "friend" of the opposite sex, the person lets their guard down and spends some time with no intention of cheating but by time the night is over they couldnt help themselves.