Isabela Merced said in an interview recently that filming her sex scene was easy because it was with another queer actor:
Bella and I were just so comfortable with each other. And also, we both have experience in queer relationships — you can just tell when a girl hasn’t kissed a girl before. You can just feel it. So both of us were already fine and comfortable, and so we could really explore. We added some kisses that weren’t in the original script. We added some moments that weren’t there just based off of our comfortability with each other.
variety.com
Could this explain why Cailee Spaeny was not cast as Ellie? She is a perfect physical match, and a much better actor than Bella Ramsey. The only conceivable issue is the fact that she is not queer. In which case, this can only be seen as a case of obvious discrimination against a straight actor for factors outside their control.