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If, instead, the love/relationship and the marriage are seen as two separate things, a trouble in the relationship can lead to a strengthening of commitment to the marriage while the relationship is worked on.
There can be a tendency to rely on being married as an emotional and even financial prop. It is all too easy for one or both in the relationship to not be concerned enough about the relationship's maintenance. If this is among the concerns of the person not wanting to marry then far from non-marriage implying lack of commitment, it will be that non-marriage is a very direct desire for the relationship to last.
People marry too often without thinking about what it means, they assign their own romantic ideals to it, imagining the relationship together and rarely actually thinking about the commitment and all possible outcomes. Even when they do think of their wedding vows and the implications, they are actually concentrating on the wrong thing (see next point).
Marriage is a social contract, entered into by two people only because they are both party to it (you don't get a marriage with only one saying "I do"). However, it is the only contract where the legal ability to break the contract has nothing to do with the terms of contract. The vows, legally enforced (in the UK), actually have no legal validity. You cannot avoid the marriage being legally broken by proving you kept to your vows and you don't need to refer to the other person's vows in obtaining a divorce. Furthermore, the terms of divorce can - and do - change periodically, so you can't even hold to the terms you initially married under.
Having confidence that the relationship will last just because you are married is proven over and over to be a harmful and damaging experience. Sure, a relationship without marriage can require more confidence, more effort from both partners but this has to be better for the relationship in the long term. Far better than having false confidence is having justified confidence based on the efforts both make toward a satisfying relationship.
Statistics can always be baffling but the last I saw of Inland Revenue (UK) statistics, there is no noticeable difference in the chance of a change in partner between those who are married and those who live-as-married. I believe there are statistics prepared for California that show something similar, too. A belief that marriage is going to lead to a longer relationship than a committed non-marriage is unfounded.