I don't believe this for a moment, and nor do you. There is no such thing as a 100% platonic relationship: sex is always there - sometimes highly obscured, sometimes right in front of you.
From a fundamental, genetic point of view you are right, but there are exceptions.
My absolute best friend in the whole world is female, and very attractive. We have been best friends for 10 years. We see eachother fully as siblings, no sexual aspect in our relationship.
The tickling and messing about is just another precursor to sex. Fore-foreplay, if you will. I had exactly this situation with a girl in halls. We were good friends and behaved identically to you - we messed about, 'slept' together wrapped up in each other etc etc. It was clear, after a while, that there was a sexual tension and we'd just been denying it. We ended up dating for several months after that.
Well, I have always found the playful, flirting thing to progress to something more, but never this sort of physical intimacy straight off the bat.
We haven't spoken in literally 6 years. Then we randomly sent a view messages on Facebook during the week, and I ended up getting invited to a day in the park with some of her mates.
If it does work into something more, that's cool, but now would be wrong, as it would be rebound, and I really am not looking for anything. I haven't been single in 3.5 years, let me at least savour it, lol