I have a relatively new partner (couple of months) and although the sex is pretty darn good, I'm missing out on getting my ass slapped. All of my old partners just did this naturally and I didn't have to ask. I'm a tad nervous asking as he is very much a gentleman.....
Well you could try wearing panties with the phrase "Spank Me" or "Punish Me" printed on the back . He may get the message. But just be careful it may be best just to get to know him a bit more first .
I think if you have been seeing him for a couple of months it shouldn't be a problem to ask him how he feels about spanking in a casual way. Maybe he's not into it but he might be willing to do it if you let him know that you enjoy it.
