I consider myself very liberal politically on social issues. I support gay marriage, I'm pro-choice. Going further beyond the standard talking points I'm pro legalisation of euthanasia, prostitution and drugs heck I'd even legalise polyamory. People should be able to live the way they want to as long as it doesn't hurt others. With that said just because you can do something does not mean that you should do it and while I am liberal politically when it comes to my personal life I definitely believe in traditional values i.e. a two parent nuclear family. While I would never legislate to encourage this (as its none of the government's damn business) Conservatives are definitely right when they speak about the decline of the traditional two parent household being a terrible thing for society. Lets look at the facts study after study show the following:
Living in a single parent family is strongly correlated with school failure and problems of delinquency, drug use, teenage pregnancies, poverty, and welfare dependency in the United States. Children living with one single parent have worse well-being in terms of physical health behaviour, mental health, peer friendships, bullying, cultural activities, sports, and family relationships, compared to children from intact families. Children of single parents, after controlling for other variables like family income, are more likely to have problems, including being twice as likely to suffer from mental illness.
With this in mind I don't think that anyone should have children before you've achieved each of the following four steps:
You and your partner should both have full time jobs before having kids. Kids are expensive and to give them a good quality of life you really need both parents to work (unless one of the parents is earning a really good wage). You almost certainly both need to be working before kids in order to get a house which leads on to point 2. If one parent decides to go part time or stop working to devote time to the kids once you have them that's fine but you need to build a solid financial base first.
You and your partner should own your own home. Children need stability above all and you really need to own your home to do this rather than constantly be facing eviction if the landlord decides to boot you out for whatever reason they choose i.e. they decide to sell your home. A mortgage costs less than renting and you need all the disposable income you can get to give your kids as good a life as possible. If you're unable to afford a house you aren't financially stable enough to have kids yet.
You and your partner should have lived together for a number of years before getting married. While you can't rule out 100% that you won't get divorced, if you're able to happily co-exist living together for a number years your chances are obviously better than if you haven't done this and if you break up during this stage no harm no foul as kids aren't involved yet and a lot cleaner than going through a messy divorce.
Perhaps the most controversial point but really you and your partner should get married before having kids. It shows that your partner is committed to you and is just the sensible thing to do as it offers more protection under the law than co-habitation does such as benefits to your surviving spouse should tragedy strike and you suffer an untimely death which help to avoid your surviving family living in poverty and if your surviving spouse works as well this helps to cushion the blow as well.
Honestly if no one had kids before following these four steps don't you think society would be much better? While less kids would be born those that would be born would be much more likely to grow up in much more financially secure, loving households which can only be a good thing for society as this will lead to happier more well adjusted kids. Crime will be lower and kids able to fulfil their potential. It would create a virtuous circle as the kids of these individuals will then be well equipped to have kids of their own grow up in similar circumstances to the way that they grew up.