This is totally my opinion based on some personal observations. That being said I feel that a lot of the reason why a number of supposed "Christian" marriages fail is because of the irrational way in which young people are taught about relationships in the Church. It is not necessarily the Biblical plan for marriage that is flawed but rather the institution of the religion of Christianity and the emphasis placed on how young people view relationships.
What I saw and experienced growing up in church was that young people are taught that sex is bad, everything that
might lead to sex is bad and that we should just wait for our husband/wife to come into our life and make everything right. Then when these young people get to be of reproducing age they are stuck in the irrational mode of self repression because they want to do all these things that they had been taught was evil. Then they get this idea that marriage is the way out, the way to be able to express these desires and they end up marrying the first promising person they meet in their church.
So basically there is little teaching of what the real issues behind all these religious restrictions are or how to go about managing relationships so that you can even find the person who is right for you. The only seal of approval for a mate is if they are a "good God fearin man/woman."
What the Church needs to do is realize that this is a different time and that they need to teach their children how to have healthy relationships. We should stop treating our children like idiots, stop "protecting" them from the world that they are going to have to live in anyway, and start teaching real life skills as it pertains to relationships and marriage as well as a ton of other things.
This is not to say that this is in anyway universal or anything. I realize that there are some churches out there that are more enlightened when it comes to these issues. I just know that from personal experience that I have seen quite a few couples go down the road of marriage in ignorance while expecting it to totally succeed because they are following the Biblical plan. The Biblical plan for marriage can and does work in Christian couples who actually understand it, understand what it means to be married, and actually got married for the right reasons. It does not mean that it is the only plan for a successful marriage though evidenced by the many people on this thread who spoke of their successful, non-Bible-plan marriages.
So what good is there in the Bible about sex, love, and marrige for non-believers? Well probably not much but it can and does make sense often for those who do believe and can apply the concepts appropriatly in their life.
The Biblical plan for marriage is not for everyone just like Christianity is not for everyone.