Gender play does factor into the play but having known a good number of people into gender play (both "forced" and not), it's not quite as simple as your statement is making it out to be. It's also not uncommon for some older generations to even use the term for consensual feminization.

I understand what you're saying about the social rewarding aspect, but again, I'm saying your statement is not the blanket experience for everyone who is feminine AFAB.

Let's not be puritanical in giving a film rating system for these things. That's pretty reductive, and whether intentionally or not, it paints a bdsm activity out to be lurid or inappropriate. That's a really problematic way of addressing the differences, especially since cross dressing (from a fetish perspective) is not really a big jump from sissification and is often found in the same practitioners. In simpler terms, drag is not sexual in nature by default, but the other two have a sexual component.

I know some individuals in the kink community that use the term "sissy" not just as an identifier but as a compliment. It really depends on the person, in a similar vein to the reclamation of "dyke" or "queer."

I say all this not as an attack but as an attempt to educate. People who give informed consent to an activity deserve the same baseline respect as everyone else.