I believe it is. If I have sex with someone who is an adult and a willing partner, I do not believe it is immoral simply because we did not have our relationship confirmed by a gov't document and a church official.
So if it feels good, do it?
You will notice I said sex between consenting adults. And if that criteria is met, yes.
Now explain how sex between consenting adults in the field doesn't eventually become a morale problem.