Phoebe can't very well tell them that Cole had been a demon and at one time the source of all evil. Which of course didn't always have to follow that their sex lives were different than anyone else, only that the line in the sand was likely further out than most. She honestly had to think if there was a hard limit and with the memories close to the surface of the 'wild west' she only gave the only one she could think of.

"No, just not into S&M but that's it." That was her only caveat and that had more to do with torture triggers and memories from warlocks and demons.