Herbert,

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 the brain;
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
a strange 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 your relationship;
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.

Courage,

Bob Kirby