Hi Adam,
A conversation has arisen in the pub like this with me previously - a discussion around general things we find sexy in other people - if you instigate a conversation like this it will be easier to start explaining it.
There's nothing unusual really about finding it sexy when someone is ticklish - it's to do with stimulating them, making them laugh, flirting, teasing, fighting - all sexy things
I would remove any association with sex from the conversation, unless you are referencing an existing relationship with a partner - ie referring to tickling as something you enjoy sexually with your current partner - it will seem less weird and easier to explain if you have an actual example to give of someone who has accepted this as part of their relationship with you, particularly if it involves a partner who is known to whoever it is you are telling about tickling.
Otherwise in general just focus on how much fun it is, how you can came to like it just through using it as a means of flirting/messing about with friends, enjoying making people laugh, cheering people up, the character Mr Tickle - all the wonderful ways of using tickling without a sexual agenda - ways that whoever you are telling will be familiar with and will have probably been on either end of.
Express how you appreciate that many people find it torture and you can understand that and wouldn't every carry on tickling if it was horrible for someone. Being tickled is a form of enjoying domination - which is nothing unusual so you could mention this.
Hope that helps
TTG
