So far, the dispute in the Church over marriage has been concerned with just that - marriage. People's promises to each other, under the attention of God, and the union they make. And, as usual, the entire question quickly gets framed as one of 'mercy'. Are we really going to make two people live together for the rest of their lives (under penalty of being considered sinning, anyway), just because they both made a promise to each other that they were too stupid to understand the meaning or at least the gravity of at the time? Come on, etc.
Okay: go with the 'mercy' logic. Say it's unfair, even unreasonable, to expect people to honor their marriage vows, on the grounds that they had no idea what they were getting into. Fine.
Now, turn around, and tell me that - marriage or not - they have to look after their kids for the rest of their lives. Even though you can (and more and more, people often do) make a child without even an oath being involved. Most marriages involve a hell of a lot of lead-up and planning in our culture, longer than it used to. Making kids is as easy as it ever was, and far more all-consuming than a marriage is. You can take some time away from your spouse to cool your jets. Doing that with a kid requires money or criminal neglect.
Tell me -that- isn't a potential recipe for a couple decades of misery too, or that people had no idea what they were getting into there either. Will someone be pleading the case to me that a guy shouldn't have to be on the hook for at a minimum 18 years of his life just because he made a several-minute consensual mistake? You don't expect him to stick around with his wife because he or she were ignorant. But sticking around to take care of a kid, -that- he understood the gravity of? To say nothing of the mother being expected to do the same?
Even if you put the 'abortion' phantom aside, this invites a tangle of questions, none of which have good answers. And the same general kind of logic which says 'Well, mommy and daddy are happier and (in their minds, anyway) better people when they aren't together, so it's okay when they split up' is going to be pretty easy to recycle for 'Well, mommy and daddy are happier and (in their minds, anyway) better people when you're not around, so it's okay that they left'. The only reason it hasn't happened yet is because SOMEone has to take care of that kid once it's born, orphanages are scary (and thus social stigma stays in place a bit more easily), and the compromise has been to treat the state as a father stand-in insofar as it will write checks and try to shake down dad for his share for as long as possible.
How long that situation can last is an open question, but I think it's safe to say it requires - at a minimum - a constant flow of money from the government to have any hope of lasting. And that's speaking purely in a pragmatic sense where we ignore, contrary to what feminists would like to insist, how screwed up these kids and households alike tend to be. Watch what happens if a severe economic downturn pops up.
But to circle it back, the question will eventually become: if the duties and obligations of marriage are treated as null and void largely on the grounds of 'Well, you didn't know what you were getting into', how in the world do you keep the duties and obligations of parenthood itself in place? Argument from 'Well damn it's going to be real inconvenient if we don't'? Argument from 'Well we don't care if it's tough, this is your responsibility now and if you can't handle it then stop breathing'?