I have worried about this myself. dd is the result of a drunken one night stand. Her dad lives on the other side of the world, with his new girlfriend & daughter 8 months younger than dd is. We were never together as a couple, in fact I didn't even know him very well, he just worked where i worked! 
Thankfully we get on very well and have good email contact, he makes an effort to send presents etc for birthdays and christmas etc and she knows all about him. (She is 2.6 so only the basics, even that she prob doesn't fully understand!)
When she is older there is no way I could lie to her given the circumstances. I see the value in keeping something like that from a child who is part of a "normal", loving family, as someone said it is irrelevent, but for dd, there is no other way I could explain it. "Unplanned" doesn't really fit either. I would have to explain that she was the result of a one night stand surely? I always think of her as the "best thing that ever happened to me" (so I will def use those words perfectly honestly, somewhere in the explanation) but in all honesty I don't think I've yet got over the mortification I felt for having been so stupid, and of course particularly in the early days there have been patches of "if only" when things got tough, particularly re my boyf who i got together with before finding out i was pg (NOT if only i'd aborted, that wasn't an option for me personally as you may have guessed (!), rather if only i hadn't got pg) - though it's not as if in spite of those feelings I would ever REALLY want it any other way. So if i am going to be completely honest with dd - and i'm all for honesty to tbh 
- it would be hard to leave out any of that. I'd imagine when your children have children themselves and start asking questions about your experiences, truths would start coming out? My mum has been very open about our early years since I had dd.
Have actually found it quite difficult putting all that into words...I have always written an extensive diary in which i'm always brutally honest, but saying it online with other ppl reading it is somehow different. Feel a little teary and am brimming over with love for my little dd! :) Perhaps i can finally move on now...