Two and a Half Men S11 E04 Clank, Clank, Drunken Skank

"Two adults. One kid. No grown-ups."

Alan and Lyndsey's illicit relationship becomes jeopardized when one of Lyndsey's man-hunting friends, Stephanie (Jennifer Aspen), asks if they are back together. The two staunchly deny it, then Lyndsey insists that Alan make a date with Stephanie so that she will believe their lie. Alan meets her for an uncomfortable date, which nevertheless leads to sex, causing Lyndsey to get mad and leave Alan again.