This is an important question, especially because it frames the cultural pressures surrounding marriage in the right way: Why don't we teach boys that they need to get married, the way we teach this to girls?I started to mull this question over, asking myself if there had been a time when the people around me sought to teach me that I needed to be married. And as it turns out, I can remember a time when people told me that I should aspire to marriage. But I wasn't a boy at the time. People tried to teach me that I needed to marry when I was an adult - once I had passed into my twenties and hadn't yet started to actively look for someone which whom to "settle down" or "share life with." After it began to dawn on family, and to a lesser extent, friends (and friends of the family) that I wasn't planning on marrying, as everyone had assumed I would, then they began trying to educate my on why I should.
Emma Green, "Wealthy Women Can Afford to Reject Marriage, but Poor Women Can't"
Of course, people understand that once someone has graduated college and moved into the working world on their own, it's a little late for the sort of indoctrination that you can get away with on children. And so the tactics were different - and all over the map. One of my friends challenged my courage, another my capacity for selflessness, an aunt's co-worker framed it as a responsibility I owed to Black women and my father pitched marriage as the answer to having to do my own housework. But it was clear from everyone that I had violated an expectation. (Something I was well aware of. When someone erroneously assumed that I had a girlfriend, I never corrected them - that people found "the idea of nonsexuality more bizarre than deviant sexuality" was quite clear to me.)
In this, we could answer Ms. Green's question as follows: For boys, that they will find marriage to be important, that one day they will need to marry someone, is simply assumed. Everyone expects that, at some point, any given male child will want to find a partner and "settle down," and so there isn't the pressure to specifically set out to teach them this necessity. But that kind of falls flat, because it doesn't answer why we expect boys to gather this via osmosis, yet girls need to have the lesson taught and reinforced over and over.
Using my own experience for this is kind of iffy, because I was child in the 1970s, and even if I could recall all of it clearly, it still be decades removed from the experience of many, but I'm going to take a stab at it. But to do so, I'm going to alter the question a bit. In my understanding, both boys and girls were taught by the society at large that they should one day marry, whether as a rite of passage into adulthood, or just so that people didn't think that there was something wrong with you. What I would say was different about girls was that for them, marriageability also entered into the mix.
When I was a single twenty-something, and everyone had an opinion about the fact that I was unattached and planning to stay that way, it was made quite clear that the ball was in my court. Especially as concerned finding a Black woman to be a partner. All I needed to do was sift through the available offerings, find The Right One, and pop the question. I was the active party, the one who was expected to go out there and chose someone to pursue and/or wed. My understanding of the expectations for women was that they should concern themselves with remaining eligible to be chosen. While my being single was viewed, variously, as something between a quirk and a serious character flaw, it was never viewed as something that fundamentally lowered my suitability as a mate (leaving aside, of course, the simple fact that willingness has something to do with suitability). That particular trait was governed more by the facts that I had a college degree, a job and a car/apartment. But even then, when I was being pressed to complete my Bachelor's degree, find a job, and have a place/car of my own, these were things that were important more for the fact that they marked me as an independent adult - no-one pushed me to finish school because no-one would want to marry me if I didn't.
And so (and I understand that I'm guessing here), I would venture that, at least when I was young, the "need to get married" wasn't taught to girls as an end in itself, but as a proof of worth as individuals and/or women. And although it has that connotation for boys to a degree, what makes men worthwhile as individuals and/or men is based on other things than whether or not a woman would choose to marry one. In that sense, I think that, regardless of what one sees as the economic or personal benefits of marriage, that we're better off dropping how we teach it to girls, rather than broadening it to all children.