I understand they are not the same. But I think an LTR has a lot more in common with marriage than an LTR has with dating. LTR and dating are two completely different entities orbiting entirely different stars, while LTR and marriage are at least in the same solar system. You can't say that an LTR that one or two partners thought was a preparation for marriage and having family together is the same as casual dating and therefore should be treated likewise. The are different rules, behaviours and perceptions.
Though, ultimately if it's not working out, it's not working out and relationship should end, be it casual dating, LTR or marriage with children. Though any relationship is not peachy, I was always of the impression that a person entering a relationship enters it with a realization that there will be issues and their commtiment towards resolving these issues will be requried.