Getting married only makes a difference to your level of commitment if you believe that being married is important. Two people who feel no compelling reason to get married do not suddenly become more committed just because they went to a registry office and signed a piece of paper.
I've got to say as well, the idea that co-habitating couples just chuck in the towel at the first sign of difficulties, whether they have kids or not, is also completely ridiculous.
Simply put, marriage is only as important as the value you place in the concept.
Financial/legal reasons are the most compelling reasons to get married if you aren't idealogically disposed towards it. DP and I have been together 14 years, co-habiting for ten (through plenty of difficult times too thanks) but if he died early, I would not benefit from his pension and as I am staying home to raise our kid (first one on the way) my own pension is going to be somewhat lacking. Inheritance tax is a borderline issue for us, depending on what level of insurance we get and whether we put some of it in trust for our kid, or leave it all to me to manage as I see fit. DP will be on the birth certificate, so will have parental responsibility. We are logged as each others next of kin at our doctors and local hospital (that doesn't preclude family members attempting to overule it in the event of something happening and as an unmarried couple it may be easier for them to do this, I don't know the exact ins and outs of this).
I personally think marriage is a dated concept and I hate the assumptions that people make about "husbands" and "wives". If we could have a civil partnership I would, but atm that's restricted to gay couples. We show our commitment to each other by planning our lives together and supporting each other through thick and thin. The idea that in this day and age our child will be negatively affected by having unmarried parents is ludicrous, it's hardly an unusual situation anymore. I'll be teaching them that anyone who judges them for having unmarried parents simply isn't worth knowing as they are bigoted and judgemental; you judge people on who they are, not whether their parents share your moral values.
Sadly due to the pension/inheritance issues we may decide to get married, to protect me in the event of DP dying/our relationship ending. I'm very much a hope for the best, prepare for the worst kind of person. If we do though, no family/friends will know, it'll be a piece of paper in a folder somewhere that only becomes relevant in a worst case scenario and that enables us to put each other down on our pensions as recipients in the event of a death. If pension/inheritance tax isn't an issue, then a well written will should protect you both.
Final point; our 30 year mortgage on a house that may well be in negative equity is a far greater (and harder to escape) commitment than a marriage would ever be.