Part of the reason for this is that for much of their existence, condoms were not actually perceived as birth control, though they obviously could function that way. In an era (like the early modern period) in which there was little stigma in men visiting sex workers, as long as they were relatively discreet, venereal diseases were passed from customer to sex worker to customer, and there was a need for protection. I have to note that this was very much conceptualized in a misogynistic way - the sex workers themselves were essentially seen as the source of the disease, and men wore condoms to protect themselves from these "dirty" women - but yes, it would have the knock-on effect of preventing pregnancies. Men did not use condoms with their wives because that would reduce their wives to sex workers.

You also have to note that prior to the 1960s sexual revolution, premarital sex did happen. I have a previous answer on sex in the nineteenth century, from which the next paragraphs were drawn:

In [the middle and upper-working classes], premarital sex was more likely to happen, in the context of a long-term courtship that was heading toward marriage ... . Although it was rarely acknowledged explicitly, there was a tacit acceptance that being seriously engaged was almost like being married. This sexual activity seems to have been somewhat cautious and relatively infrequent, not occurring until the courtship had been going on for some time, and not happening that many times. Birth control methods other than the rhythm method and withdrawal were not really available to this class, and they knew that they were risking an unplanned pregnancy - and while it wasn't the worst thing to go to the altar pregnant, it was suboptimal. In breach of promise suits (usually women suing men for promising to marry them and then not doing so, but sometimes the other way around), it was typical for couples to only sleep together a couple of times in the latter half of a two-year courtship and engagement. They also had little opportunity for the amount of sex we now see as normal in romantic relationships: like elites, young people in this group were often with family or friends. Chances were few and far between, so couples would lock themselves in the parlor after the family was in bed, or try out the home they were planning on having when married, or wait until their parents and siblings were all away ... It was difficult. In The Long Sexual Revolution (Oxford University Press, 2004) Hera Cook speculates that experiences of abandonment in the nineteenth century among the upper working class caused successive generations of women to be more and more suspicious of men who wanted sex outside of marriage, creating a culture of "respectability" by the end of the century that contrasts with the stereotype we have today of "prudishness" going along with the stuffy elite; she also notes that as urban communities settled in the later nineteenth century, the poor were better able to form strong social networks with more supervision and surveillance of the unmarried - but whatever the cause, the illegitimacy rate fell from 67% to 39% through the period, almost halving. All that being said, this was not really teenage behavior, because most of the people who did this were in their twenties, or possibly very late teens - the average age at first marriage in England was about 23 for women and 25 for men. Actual adolescents in this social stratum were still not supposed to be sexually active, and if a girl were found to be so, either through gossip or having an illegitimate child, it would have been shameful for her family.

Among the poor, sexual relationships appear to have been similar in that they were serious rather than casual, but they were as likely to occur from economic necessity as from preparing for marriage: the changes caused by the Industrial Revolution made it harder for a single woman to support herself, so cohabiting with a man outside of marriage was seen as a possible way of getting a place to live and food to eat while earning pennies. However, this could be dangerous - if she became pregnant and he abandoned her, she was worse off than before, and if her birth family couldn't afford to take her back in with or without her child, she would end up in the workhouse or living on the streets. But serially monogamous sexual relationships were common, essentially like marriages followed by divorces - but in a period where divorce was next to impossible for the poor to achieve, it was simpler to not marry in the first place. (That being said, a number did marry-for-real and simply bigamously remarry if deserted by a spouse.)

Around the turn of the century, a dating culture developed which began to dismantle the engagement barrier and began to normalize more casual relationships, though again in the lower-middle/upper-working classes for the most part. The following paragraph is a quote from another answer of mine:

In the 1880s and 1890s, women started to join the workforce (as factory workers, as saleswomen, as secretaries, as waitresses, etc.) in larger numbers than before, and to do so to support themselves rather than their families - they were, in fact moving out of the family home and into their own rooms in boardinghouses. According to Moira Weigl in Labor of Love, by 1900 half of the population of American women were working outside the home, and were free and able to meet young men they didn't already know. However, since they were paid half (or less) of what men in similar positions were being paid, they also had very little discretionary income - there was no chance to go out and do anything fun unless they were being shown a good time by a young man, and just as this implies, this could be a slightly seedy transaction. A "charity girl" was a young woman who was "treated" (taken out) by a man in exchange for sex, legally safe from being prosecuted for prostitution, but considered essentially the same as a prostitute by the parts of society not involved with the dating world. College students were in a similar but much better situation. Like young working women, they had independence from the family home and were plunged into a world with many young men they could introduce themselves to; because they were at college, though, they were usually from an affluent enough background that they weren't so destitute. Both groups would go out to dance halls, music halls/vaudeville, and movie theaters instead of the family parlor. In the 1910s and 1920s, the concept of "public courtship" began to seep into ordinary late-teenagehood.

By the 1950s, illegitimate teen pregnancy was at an all-time high, so the popular idea of the 1960s sexual revolution as liberating people from the prudish past is somewhat misleading. What the Pill did was give the women who had access to it the ability to control their fertility, rather than having to rely on a male partner to purchase and wear condoms, and as a result even upper-middle- and upper-class single women, who had the most to lose socially from pregnancy out of wedlock, to partake in the emerging culture of casual sex, completing its normalization.