I'm exactly the same since I was little it has always fascinated me and I've always longed to try things. It wasn't until I was much older and met my current partner that I could start experimenting. I too had a great childhood so maybe its just something your born with? Or developed a fetish.
Want to be kidnapped and kept captive ... Tied and gagged .
I couldn't point to a starting point. I just remembered a scene in Superman where Teri Hatcher was tied up, and was just mesmerised by it. It just took off from there.