Ideally marriage (and separately, parenthood) would be limited only to people qualified to do the job well. Fat chance ever making that happen.
If we somehow did make marriage hard to get, many hetero couples would be forced into a twilight zone akin to what the gay folks put up with for so many years: A committed relationship under a variety of names without the benefit of official recognition. Which is all sorts of wrong as a practical matter. Thank goodness we’ve mostly come to our senses and let the willing gay folks get the full legal recognition they deserve. Taking that away from anyone now would be be a step backwards.
Forcing large numbers of would-be married couples into those sorts of “halfway house” arrangements would not do anyone any good. The cure would almost certainly be worse than the disease.
Interestingly, AIUI, New Zealand does it the opposite way. If you cohabit for 6 months, you are legally married. No ceremony, no license, but the couple has transitioned legally into the exact same status as the more traditional civil and/or church wedding / marriage.
Which is all about recognizing that as people spend more of their lives together, they develop common obligations to the world and to each other whether they want or like that or not.