I'm not saying that getting married will make someone mature or adopt better behaviors.
Getting married, if nothing else, changes your legal and financial status. Usually, it changes your social status. People like me also think entering into holy matrimony is a spiritual change.
I suspect that with some guys, especially those shacking up, the thinking is that getting married won't really change anything, except that there will be an expectation of sexual exclusivity, which presumably had been in place already.
By law, power shifts upon marriage, especially if a groom is earning more than his bride (as most do) and cares about his hard-earned money. Because of that shift in power, along with other aspects, it is quite often the case that everything will change.
The guy who expects that life will continue much as it did before is likely in for a rude awakening.
His time is no longer his own. Here comes the honey-do list! Wives schedule and plan most of the social engagements. Chances are, he will be seeing less of his unmarried friends. This is no accident. He will definitely see less of any attractive, unmarried females from his social circle.
His money (by law) is no longer his own. Wives make most of the purchasing decisions.
She will probably work to change certain habits of his, even if she never expressed displeasure with them before marriage.
What he wears might change.
What he drives might change.
Where they live might change.
Things kept in the home will change.
The content of the media he watches or to which he listens might change.
She will probably get involved with decisions about his career.
The sex will probably change, both in frequency and quality. Generally, that will not be a change for the better. The exception, of course, is if they abstained before marriage, in which case the sex will be better by default.
Sometimes, these changes are initiated by the husband, because he knows it is expected of him and that's what a "good husband" will do. In other marriages, she will explicitly or implicity request and take control. The changes will be for her benefit. If he fails to go along, or tries to make changes that will make things better or easier for him, he'll be called "controlling".
The husband may notice the changes as they are happening, or he might be like the frog in the kettle, and wake up one day and wonder "What happened to all of the time I used to spend with the guys? What happened to all of the things I liked to do? What happened to the sex? What happened to climbing that career ladder?" And he might not even care. If he expresses displeasure at the changes, his wife, her friends, the women in the extended family (oh yes, you make her family in-laws when you marry, the folks at church, the counselor will call him, in so may words, a selfish bastard.
These are just the changes that come with marriage. Having children changes everything, again.
In many ways, that is as it should be. While, as a Christian, I do not believe my life is my own anyway, it becomes someone else's in a very a visibile, practical way upon marriage, and upon becoming a parent. the problems come in, for me, with society holding men to this in a way that it does not hold women to it. Masculinity is stripped away. Guy time is largely lost. Both law and the social climate favor the wife and mother over the husband and father, and in ways that are at the expense of the husband and father.
I'm generally happy with my life. But happiness is a choice. I've been meaning to write about that for a while, and a separate entry will have to wait a while longer, but the gist of it is, there are so many things I'd rather be doing with my time, including things that I used to do that I have let fall by the wayside. I can and do still choose to be happy. But is that (muliplied by millions of others in like situations) encouraging to the next generation of men? The 50 percent divorce rate, Michael Medved insists, is a myth. Okay, so let's say that it is just 25 percent. Would it be unreasonable to say that at least 25 percent (or 1/3rd of the remaining) of the married people who don't divorce are unhappy much of the time? That leaves about 50 percent of the marriages as "happy". But how many of those are happy not because they are satisfied, or have the life they were working to acheive, but rather because they have decided to be happy? So the umarried man considering marriage is facing the reality that, odds are, he's either not going to he happy, he's going to be divored, or he's only going to be happy if he stops caring about his goals, his interests, and his desires. That's not exactly encouraging.
Of course, the flip side of this is the assumption that having more personal autonomy will mean more happiness, and of course that isn't necessarily true.