Assume you really connect with a guy. You like him. You've waited long enough for sex to be comfortable. He meets all your requirements for a LTR or more. Then you have sex.

What could make you decide that the two of you are so sexually incompatible that you need to call things off? How many encounters would it take you to decide that he can't be trained out of this issue or it it cannot be lived with or fixed in some way?