I didn't say it didn't suck. I said this particular kind of suck sucked less then than it does now.

A couple that is dealing with a kid that was born when they were 16-19 is far less of a burden in 1958 than in 2018 assuming because how society expects them to handle it then vs now and how much of a burden we consider it then vs now.

In 1958 the expectation was that you get married, settle down, raise the kid and pretend to be a normal couple who aren't just together for the kid. Even if you don't really love each other you at least get to raise your kid together and manage your time and money like a normal couple. This is ball and chain to both parents and certainly not ideal but it's a lot easier to do when that's what society expects you to do. Yes, you're settling, yes, you're stuck with someone you probably don't love anymore but that sucks a hell of a lot less when it's something that you're expected to do with some level of success.

In 2018 the expectation is that the couple splits. Raising the kid while apart is a massive handicap on both parents. They're expected to still manage to have careers (which probably involve college), find new partners and generally pursue life like everyone else. With a kid to raise these things are much more difficult and complicated. Even if you make the generous assumption that parents with shared custody cooperate with each other in order to raise the kid the overhead cost (time, money, frustration, pick any metric you want) of separated parents trying to coordinate to raise a kid is much larger than a couple working together. If one parent is completely out of the picture or the couple has shard custody but doesn't want to work together then it's still a bigger drain on both (money for one parent and not getting to raise your kid, everything else for the other). Neither case does the kid any favors.

Basically in 2018 we expect couples who have a kid in high-school to do it the harder way and society's expectation that they do it the hard way makes it even harder on any couple that tries to raise the kid together (contradicting social norms is hard to do because basically nobody around you will support you).

Economics aside, raising a kid with your high-school sweetheart that you no longer love is ball and chain on both parents but it's a smaller one if the kid is born in 1958 than in 2018 because per 1958 norms you're expected to get married and tough it out. Per 1958 norms having a kid in high-school not seen as the end of the world, it's seen as something unfortunate that happens to a non-negligible fraction of people and that people are expected to be able to deal with somewhat successfully.

Sure, you can hair split about the economics of raising kid, opportunities for women to have careers, quantify how bad it is to be married to someone you don't love vs having shared custody vs writing a check every month. The end result of all the hair splitting will just reflect the opinions and cultural norms of whoever doing the splitting.

I'm saying that even if you ignore all that having a kid sucks less in 1958 because how much it actually sucks is determined in mostly part based on how much we think it should suck which is basically determined by the society we live in and in 1958 we didn't think having a kid at 18 sucked that bad as we think it does in 2018 so therefore it sucked less for the people who had to do it in 1958.

edit: it should go without saying that this assumes parent quality is competent across time. Of course you could make either situation look like the obvious winner but comparing good parents to deadbeats.