Yes, it is acceptable, because two people who have feelings toward each other should express themselves, regardless of marital status. It is morally acceptable, as long as the parties involved have both consented, and no one gets hurt. There are too many old-fashioned beliefs that need to fall by the wayside. There are too many serious indiscretions in this world to worry about something like pre-marital sex. More serious indecencies have been going on behind the closed doors of "religious" institutions for centuries.