OP, I'm also in the same position as you and ButWhyIsTheGinGone. I work with children, love it, but love my adult life too. I'm looking forward to being an aunt but not a mum. My DP feels the same.
I was absolutely gung-ho for sterilisation when I was about 28-29 (only 31 now so not patronising you!) and discussed it long and hard with my DP. He wasn't keen at all as he felt it was too drastic a step to take. So we shelved it for a few years. We both still feel the same re no kids but I'm becoming more ok with the idea that I MAY change my mind. On balance, I suppose I'm glad I didn't go ahead with it, and I'm not planning to in the future.
I don't think you are being unreasonable AT ALL - in fact, I think you are being extremely responsible and forward-thinking and good for you. I would imagine you would have to jump through many hoops to get sterilised on the NHS as a young child-free woman. For what it's worth, I use Mirena and while insertion was hellish, I have been bleed, cramp and pain free for over a year. Of course, Mirena is not right for everyone, but there are long-term reliable contraception options available.
And by the way, I agree with other posters who say that hanging out on Mumsnet is NOT a good way to sample parenthood - for all sorts of reasons! Spending time with real-life children is the only way to do that. And I think it bears repeating that if you decide parenthood is not for you, that is a perfectly legitimate decision. it's not always the easiest decision to live with, but you have to be honest with yourself and do what makes you happy. Good luck!