In the wiretaps, the Madame and “Kristen” discuss how Client 9 would want things which “aren’t safe” and how Kristen would deal with that.
In context, it was clear to me that they were talking about condom use, rather than some of the other stuff thrown around here. I’m not sure where Heidi gets anal from - but, I suppose, she’d be in a position to know some stuff.
She’s thinking of “strap on” portable gear.