Did he ever ask for more women? That's the only reason I can think of that she would use the creme on him.
I guess the big question is if using the stuff on him was Laura's idea, or Kurt's. This update inspired me to re-read the whole series, and I just realized from the first issue that it seemed to be Laura's idea to try the stuff in the first place. Given how much Kurt has been in charge this whole time, I could have sworn this whole thing started as his idea.
So maybe Laura wants a "bigger" cock (having been so enthralled by Kurt's) and since she's now with her boss she'll make him "bigger" as well. Or maybe Kurt wants another guy with the same dominant streak out there to continue the work of corrupting more women...