I've asked Francisco's question of myself many times and the answer is simply "nothing", there is no advantage for a man in being married and if anything you assume more responsibilities, liability and accountability than even if you were in just an LTR. No one can answer this sufficiently and this is the question that all women dread, so they're forced to create social contrivances that rely on a guy's sense of lonliness, isolation and powerlessness with regards to intimacy that are fostered throughout a man's formative years. Like most feminine operative conventions, this is a sexual acceptability culling or filtering process in order to encourage (really shame) a man into living up to a perceived ideal in order to ensure long term security, while keeping him just confused, frustrated and insecure enough to make him believe that he's powerless with women because they're this unsolvable mystery.
This may be changing now, but, most men I know, only ask themselves "what does a man really benefit from marriage" after they're married. Some never do and proceed into marriage number 2, 3 maybe 4 all the while still trapped in this Matrix of the expectation of being an 'empowered' man, but not so much as to be independent of a woman's intimacy. The main parodox in all of this is that for a woman to achieve her fantasy of long term security, a man must necessarily give up, in part or in whole, his own ambitions and autonomy, or in the very least limit them. This is, for the main part, a sacrifice that women can never fully appreciate because from a very early age they've been socialized to believe that not only is their idealization of long term security a beneficial entitlement due them, but also that anything less, any man unwilling or unable to provide that security, by definition is not a man at all. The sacrifices of autonomy and ambitions a man makes in order to provide security (or at least perceived security) will never be appreciated in the feminine because it's been so socially conditioned in both sexes that it's simply and unquestionably something a man "ought" to do.
When you introduce a very valid question like "what does marriage really benefit a man?", it shatters this convention so the men who do ask it HAVE to be marginalized, and increasingly this comes from both genders now. As I suggested, most men who come to this conclusion are married themselves and are experiencing this imbalance or are divorced. This makes their marginalization very easy; they're made to be bitter, resentful, mysoginitic, or finding a way to vent their marital frustrations, etc. In any case, under the current social attitudes prone to ridiculing anything masculine, both men and women don't even need to speak the argument to marginalize a man asking a question like this - the knee-jerk automatic presumption is that he's bitter, etc. and thus the valid argument is avoided and makes those avoiding it feel better about themselves.
It's the man that asks these questions prior to marriage who's the real threat and needs to be "re-convinced" of the merits of marriage. He needs to be shamed back into accepting the convention, so he's called a 'Playah' or unromantic or told he's too immature to see that he gets love and intimacy, things he could get outside of marriage, but is told that the quality of them is inferior to the same in marriage - "boy, are you missing out." Or worse still, he falls prey to an "accidental pregnancy" and is shamed into "doing the right thing."
Remember, all of this is coming from a man with a fantastic marriage. There are a lot of internal rewards in marriage, but I'll be the first to say that I suspect I could've also experienced them outside of marriage as well. I should add that there are many experiences and rewards I won't enjoy because I am married. I also firmly believe that raising children in a married relationship with a strong, positively masculine father and positively feminine mother offers the best environment for them to develop a good understanding of gender. Parental investment needs to be shared and while it's entirely possible to raise children by a single parent, this will always developmentally and psychologically hobble a child in their gender knowledge. So, to be specific, one could still have children outside of marriage, but when a husband and wife play on the same team and both are mature, with a healthy understanding of their gender roles, then I think this is one advantage being married affords a man in a more or less ideal marriage.