Beth Looked as though she was quite happy with Gwendolyn. The robot caught her interest and arousal. She only really balked at going far with Gwendolyn because she perceived Gwendolyn as being overly 'used' by Morty. ...
I might have written that scene slightly differently ... Beth puts Gwendolyn in the bath, washes her clothes and tries them on. She then drags the clean G back to Morty's room. While admiring herself in G's outfit, Beth accidentally switches on the helmet and gets horny seeing Morty fuck G. She decides to have some fun with G herself. She puts the helmet back on G, switches her on, and gets G to go down on her. Just as Beth is cumming, Morty and Morticia return. Before Morty enters his bedroom, Beth slides G under the bed, and puts G's helmet back on her own head, and pretends to be G, as she is still wearing G's clothes. The scene then plays out as normal.
I totally see a moment Beth gets the notion to play Gwendolyn again expecting Morty to take his frustrations out on her only to get maybe a Morticia or Morty lost a bet and Morty and Morticia get jiggy with "Gwendolyn" scene.
Unlike Morty, Morticia would realise straight away that it's not Gwendolyn. I fancy a scene, perhaps after admitting that she was G the previous time, Beth asks Morty, to fuck her the same rough way he fucks G. I think this would happen in the future though, as Beth fully embraces that she wants to keep having fun with Morty.