I should make clear that the real antithesis, in my view, for the single-state is the religious life, not marriage, usually. It is in light of it that I have primarily weighed my criticisms of this new fade of single-dom. The greatest difference between the two is obedience.
Of course, there are degrees of self-directedness even in the most rigorous of religious orders, it would be dehumanizing were it not so, yet the degree to which the single-stated person only answers to himself is hazardous to spiritual perfection, or improvement in any way at all. I know what I am talking about, I spent 24 years in that state. The single person can take or leave his spiritual director's advice, he can do and read whatever he likes, follow any type of devotion or spiritual practice, etc. Sure he does objectively good things with his time, talent and treasure, but he does them according to his preference. When do unwanted things occur? Unwantedness is the rule and not the exception within God's providence (may His name be praised), and so our spirituality should imitate this.
Personally, I thought I was a very godly person as a single. When I got married and had children I saw what a facade that was. It is hard to do what you don't want to do, very hard. Virtue comes about through exercising a contrary inclination to the good. If you never exercise a truly contrary inclination you will not become virtuous. Simple as that.
Of course, for those who are burdened in many ways by infirmity, who have a deep desire for marriage, priesthood or religious life, their single-state is God's will, and is contrary to their personal inclination. In that case, it will be a means for sanctification if lived with surrender and joy.
When you were younger you dressed yourself and went where you wanted; but when you are old you will stretch out your hands, and someone else will dress you and lead you where you do not want to go. John 21:18I can think of no better way to describe maturation.