Teenage pregnancy is entirely dependent on the situation. If the person has decided thats definitely the route she wants to take in life and she is mentally mature enough to handle pregnancy and motherhood, as well as having the correct resources for a child, then why not, although having all three of those things seems unlikely. On a personal level, I wouldnt want to be a father until at least 30, because that will give me the time to do all the other things in life other than create; even then it wouldn't be a priority for me.
As for Teenage Sex in general, I dont have much of a problem with that, but once again i would stress that both parties should be emotionally prepared for it.
Sex and pregnancy without marriage? I dont see any moral, emotional or physical reason why marriage is necessary for sex. Although, once again I think that the couple should be financially ready and responsible. What difference is there between having sex and having sex with a certificate in a cupboard somewhere?
tl;dr version of what I'm about to say: I agree with this person.
I come from an upbringing and stuff where I don't even approve of marriage, I think that it is a pointless act - if you're going to be with someone for all your life, you will, regardless of whether you have a ring and a piece of paper to say so. Marriage these days is becoming more pointless, with the divorce rate getting higher and the costs of divorce getting lower. Who's to say that the person you have a kid with is gonna want to be around you forever? Some people say that the way marriage is becoming is a bad thing, but I believe it's better, people are more free to do what they wish which, from my observations, leads to happier people.
I also know a lot of people who have single parents, and in my experiences, they're closer to their parents than the ones who have both around the whole time. So I don't think that you even need to have both parents to raise a child (though that depends on the situation and stuff). I don't see why having a child before marriage is such a big problem, I don't even plan to get married but I still want children when I'm older, so yeah, I'll be having kids before I'm married I guess.
As for teenage pregnancy, I actually disapprove pretty strongly. If they're below 16, I think it's awful, after that I guess it depends on what type of person they are and what they want to do. I know one of my friends is 17 and she would be happy to get married next year and have kids and stuff, but that's because all she's wanted is a family and she never really had any big dreams or anything, so for her to have kids at 17-20 wouldn't spoil her life or whatever - it's what she's been planning for. I know a lot of other people who want to go to university and get a good job or travel and broaden their horizons before having kids, so for them to get pregnant would ruin that for them. I know that plenty of people go through uni with kids or while they're pregnant, but to me uni seems like it should be kind of... the inbetween period between being a 'young adult' and being an adult, having kids kinda skips that transition and you just end up becoming an adult straight away.
Ultimatly I think it's up to the individuals though, I wouldn't criticise someone who is unmarried and 20 years old with a five year old if they were able to look after it and it was happy and stuff any more than I would criticise a married 30 year old with the same five year old, it's up to how they bring the child up to make a difference in this world and to be the best that they can be - that shouldn't depend on their parent's age or marital status.