That would insinuate that she's there to put Kutcher in line, rather than to pick up the slack left behind by that August kid.
Charlie got away with being a letch because he's old, and doesn't know any better.
Jake got away with it to a degree because he's an idiot, and doesn't know any better.
Alan does not get away with it.
Using a lesbian as a sexual predator allows the old white male writers a loophole which affords them to continue using women as interchangeable sex furniture without falling prey to the complication of their hero being regarded publicly as a vile cretin who should know better.