Is it ever not a sin to masturbate?

The question is essentially this: if a husband and wife are separated
for a long period of time, and have each other's permission to do it,
and can do it without pornography or lustful/immoral thoughts, would
that mean it is not a sin to masturbate? The most common situation in
which this question arises is that of spouses separated due to service
in the military. It is true that, if a person is used to having sex
and a significant amount of time passes without sex, sexual tension
increases. This physiological sexual tension can make it more
difficult to resist sexual temptation, whether in the form of adultery
or pornography. Masturbation can relieve this tension. So, what about
this situation? Would this be an instance when it is not a sin to
masturbate?

The best answer we can give is "perhaps." Having your spouse's
permission would turn 1 Corinthians 7:4 into a non-issue. There being
absolutely no pornography/lust or immoral thoughts/desires would
remove the clearly sinful aspects that are almost always linked with
masturbation. But, with this situation, a very important question is
being overlooked. What is the alternative? What would happen if a
person in this situation did not masturbate? To say that he/she would
be incapable of resisting temptation is to neglect the power of the
indwelling Holy Spirit (1 John 4:4). The Bible instructs us to flee
sexual temptation (1 Corinthians 6:18, 10:13; 2 Timothy 2:22). The
Bible does not tell us to find ways to make the temptation less
powerful.

So, while it might not be a sin for a person in the above situation to
masturbate, the Bible tells us to make decisions with more certainty
than "might," "maybe," or "perhaps." Romans 14:23 says, "Everything
that does not come from faith is sin." Romans 14:5 indicates that we
are to be "fully convinced" before we do, or do not do, something. The
fact that "is it still a sin?" is even being asked is proof of a lack
of assurance and evidence of not being fully convinced. Asking a
question like "Is ______ not a sin IF…?" is dangerous at its core.
While "if something is a sin, it is always a sin" is not universally
true, it is much more biblical than trying to find situations in which
a sin is no longer sinful.