The real reason is actually that men in relationships find it...

The real reason is actually that men in relationships find it very stressful to 'keep the woman happy'. This causes elevated levels of eg cortisol which will shorten your life span. There was a huge study a couple of years ago on nuns and monks in Germany and they discovered that there was no difference in life span between nuns and married women whereas monks lived about 5 years longer than married men. Stress in relationship has seemingly no negative effect in women in terms of life expectancy but it is quite deadly for men.