I'm almost frightened to join in this one, but, if I'm honest, I wouldn't be overjoyed if either of my dds wanted to be a mum at 17, just because I do think there are so many other things they could be doing.
It's all very well saying that you can study/ travel/ go out with friends once you have children, but for me, since I've had my children, it's all been about them. They get new clothes, parties, school trips, I go nowheer and get nothing, and I truly don't grudge them that. It costs a lot of money to have children, and takes up pretty much all your free time (in my experience). To be a parent seems to mean giving up selfish pleasures. When I was younger I was able to be completely and utterly selfish- I did what suited me, and didn't have to consider anyone else, noone was relying on me. It was great. Having children is also great, but I know I will never really get that freedom to do just what I want back. If they go to university at 18, I will no doubt be supporting them financially and emotionally. If they did have a baby at 17, I would be supporting them likewise. I think the freedom to be completely and utterly selfish is something that everyone should be able to experience for some period of their life, and I would be sad if my dds (or ds) didn't get to have that.
Having said that, a baby is never an unmitigated disaster, and would never be unwanted . I suppose the difference is whether it was planned or not. I'd actually probably find it easier to be understanding if either of my dds had an unplanned pregnancy, as we all make mistakes, and, as someone pointed out upthread, anyone who is sexually active, whether they are using contraception or not, has to be aware that there is always a possibility of pregnancy. As someone who was sexually active at 17 (despite my mother's draconian view on the subject, and despite respecting and loving my mum deeply) it would be hypocritical of me to berate any of my children for doing so.