Merely pointing out that the decline in marriage in the US has nothing to do with increased shootings as the OP claimed. In fact there is a positive correlation to marriage rate and homicides by firearms. Correlation is not causation, but it does debunk the op's assertion. 

Now there are some numbers that indicate that it is in fact more dangerous to be married AND have a gun in the home. https://www.jhsph.edu/research/centers-and-institutes/johns-hopkins-center-for-gun-policy-and-research/publications/IPV_Guns.pdf 

"A study of risk factors for violent death of women in the home found that women living in homes with 1 or more guns were more than 3 times more likely to be killed in their homes. The same study concluded that women killed by a spouse, intimate acquaintance, or close relative 
were 7 times more likely to live in homes with 1 or more guns..."

So statistically if one want's to keep ones spouse safe, one should not keep a gun in the home.