Aye, children are a responsibility for two parents to bear. I think the strain that having children exerts in their formative years is probably the most testing time for a young couple. I'm childless myself, but even seeing the impact it has had on friends and family leads me to believe that the sheer exhaustion, and consumption of personal time, that comes with bringing a baby into the world can't be overestimated. Some younger couples are woefully unprepared for the effort it takes to raise a child these days. I wholeheartedly agree that two parents is the ideal environment for a growing child, though.
Your point on long-term relationships is right, but only so long as that exchange exists. People change, probably more extremely and unpredictably than ever, and the parameters for their love change with them. I'm with my girlfriend just over 3 years now, and I can honestly say I love her just as much as I ever have. We agree on the important things (neither of us want children, which is often the proverbial spanner later on) but I can't possibly account for the people we might be in ten years time. I suppose that may seem defeatist.
I think my point is that I don't believe love works independently of outside factors and that people's unrealistic expectations of it (gleaned from films and pop culture?) do far more harm than good. Longevity is too easily celebrated, while the quality of love is largely ignored.