I think one reason for the scene might be to strength the parallels with Dany in A Dance With Dragons. Both Dany and Cersei are two women who have fought difficult battles to place themselves in power in a man's world... only to utterly suck at actually ruling. Dany ALSO has a close intimate experience with her female bedwarmer. The difference is that while Cersei forces herself on Taena, it's Dany who is pressed upon by her handmaiden. In a way it parallels their deficiences: Dany is innocent and naive about the necessities of ruling, while Cersei is an overreaching brute.
But I think the major reason is simply to highlight certain aspects of her character.
- Cersei is greedy and controlling. When she wants something, she simply reaches out and grabs it.
- Cersei is, for the lack of a better word, deviant. She manages to break pretty much every rule in the Faith about sex, which inevitably is the cause of her downfall.
- Cersei is rash and foolish. Remember that the Merryweathers are bannermen to the Tyrells, Cersei's sworn enemy. Indeed, a common fan theory is that Taena is a Tyrell spy, who basically tricked Cersei into moving against Margaery. Only hours earlier while in bed, Cersei even considers the fact that Taena could be informing on her, but initiates sex anyways.
Also, if you buy into "Taena is a spy" theory, it isn't that unlikely that their little tryst could serve a narrative purpose in the latter books. Cersei's storyline is far from over.
I expect the only way to get a 100% canon out-of-universe explanation would be to ask GRRM himself why he included the scene. And frankly, it's not all that pivotal a scene to warrant an explicit explanation on his part.