The truth is, yes, it is a sin, irrespective of the circumstances. The
reasons are much deeper than a two minute answer, but in short, it violates
what God intended for our sexuality.
I can totally empathize with your struggle, as any man, married or not, knows how
difficult it can be.
It is a fact that men produce a large amount of sperm, and when we do, sex is on
it can even be uncomfortable physically to hold it. The urge you
experience is natural, and not necessarily a sign of unnatural lust.
Physiologically your body will cope with the wait for sex by eliminating
excess semen post urination or in nightly emissions, otherwise known as wet dreams — something not just reserved
for adolescent boys. That is not a sin.
As long as you focus your sexual feelings/actions
on your wife you'll be doing the right thing.
You can't necessarily control a sexual
dream and the object therein, but it can be a good indicator of how well you are
doing in your internal transformation toward holiness. If you find you have
sexual dream, toward someone other than your wife, it may just mean you need to practice
even more outwards signs of love toward her to overcome the selfishness that still
lurks inside — consider it a gift of insight. You can even share with your wife
those urges while
you are away, building the anticipation of your coming together. It may strengthen
she will know how much you want her — and yes, women
do like to be considered the object of
the sexual desires of their husbands despite
what some feminists would tell you.
All this being said, God would not ask us to do something and then make it impossible
to carry out. It may take an heroic effort at times, but with His grace,
you can succeed.