Could anyone provide any supporting evidence to indicate that this may have been true? It would be especially helpful if this information could indicate whether this might have been due to mental and physical fatigue, or if it was attributable to other factors.
You didn't specify a particular service or a particular country. But I am going to go ahead and say, for the most part, no.
Within three years of the end of the war, the U.S., Japan, Western Europe, Canada, and Australia had all experienced a "baby boom."
Some of this was timing: People postponed marriage and childbirth due to the weak economy in the early 1930s, and then again because of the war. They resumed when the men returned. At the same time, the younger generation got married at a higher rate and at younger ages, helped in part by strong economic conditions especially in the U.S. and benefits offered in the GI Bill. Either way, returning soldiers were clearly marrying and having families in significant numbers.
The Soviet Union and Eastern Europe did not see a boom. But they had seen population loss and displacement far worse than countries on the Western Front. Also, the Soviet Union experienced a severe famine starting in 1946. So, even if veterans were ready to settle down and raise families, things were much more uncertain. But even West Germany had a baby boom starting in the mid-1950s.
No doubt, the experience of modern warfare on the scale of the Second World War is a traumatic experience, and many returning soldiers would have had trouble adjusting to civilian life. But there doesn't seem to be evidence that the scars of war were leading them to shun marriage and procreation.
This may have been true of INDIVIDUAL soldiers. But for the majority, it was the opposite.
When American "GI's" went home, they were treated like heroes. The country passed the GI Bill and Veterans' Administration just for them, to get them started in civilian life. The country was also in a celebratory mood, which included a lot of marrying and babymaking. In this context GIs were the most eligible men in the country, and the world.
A similar (if less pronounced_ thing took place among the soldiers of many Allied, especially English speaking countries. This led to a Baby Boom in much of the Western World.
Not all obviously. The world is full of children and grandchildren of WWII vets (most famously perhaps the current President of the USA, who often talks about his Grandfather serving in Patton's Army in Europe).
However, it would certianly be the case that some did not. I'm unaware of any studies showing that to be more common than in non-war participants. However, if it were, I'd probably attribute the difference to Posttraumatic Stress Disorder. According to the folks at wikipedia, this condition results in a lot of behavior that wouldn't exactly make the sufferer a good social partner, such as:
Estimates for current Iraq and Afghanistan veterans have shown rates of PTSD as high as 20%. Supposedly, the likelyhood of a person getting it depends in part on how traumatic the situation they are put in is. So assuming human beings haven't changed significantly in the last 100 years, I think its fair to estimate that the rates were probably similar for WWII combat veterans if the conditions were equally intense.
The lack of treatment back then couldn't have helped much. During WWII it was still termed Shellshock, or Combat Stress Reaction (CSR). Reports were that about 4% of veterans had it, but there was a lot of stigma attached to it, so it is likely it was underreported.
I was watching a History Detectives just this morning that was tracing a WWII bomber pilot who suffered from the condition. The military historian they interviewed said that the standard treatment for it at the time was literally a couple of days off and a bottle of whiskey. (!)