How to make girls do it?

The best you can get is them making out together. Unfortunately they just don't possess the "have sex on..." command available. Only male sims can initiate the "have sex" commands.

On the other hand for gratuitous lesbian content that's still pretty cute to watch, simply have Hef walk up to a girl and select her. Choose "commands" then "ask to follow" and walk up to the girl you want her to make out with. Select girl two and go to "command" then "make introductions" and choose "romantic conversation." You may have to repeat this process once or twice to get their romantic relationship level high enough with each-other to make out, but the rewards for persistence include; girls talking to each other then spontaneously getting topless, kissing, hugging, intimate caressing wherein the girls fondle their behinds, giving each other massages if the massage table is nearby, dancing with each other if there is music playing (NOTE: different dance animations when in the clubhouse on the dance floor), and of course, some seriously heavy, drawn out and extended make-out sessions which include the girls burying faces in cleavage and getting pretty hot and heavy.

I do agree that it really sucks that they don't have proper sex with each other, however unless you can get your hands on a hack or cheat I don't know about, there isn't any way to fix it.