I don't have a problem with marriage being promoted as the ideal, and I was brought up by a single parent, have been a single parent, and now my dc have a step dad.
IMO, having two loving parents who live together and respect eachother, and are preferably married, is the best way to bring up children. That doesn't mean that i think that other family set ups can't be everything they need to be to create well adjusted and happy children. Nor do I think that marriage prevents there being problems present that aren't good for children.
But I do think that tomorrows adults should be trying to create the ideal family circumstances for their children, because the alternative is that they think that being in a loving and committed relationship is not that important before having children, and problems will arise from that.
Being a single parent is not ideal for the children. Look at the numbers of people on her that have to worry about their ex's having contact, chasing maintenence money, have step children that come to stay without the right clothes. If the people that chose to bring these children into the world had done everything possible to ensure that they did not have children until their personal circumstances were right, then a lot of unhappiness could have been avoided. I include myself in that.
I think my children have done fine with me and their Dad not being together, but I think I would be kidding myself if I were to say that they wouldn't have preffered to have us together.