Yeah, I've been spending some time investigating this and asking around online LGBT communities, and it does seem exclusively for males.

It seems both hetero and homo males can be sissy, and that it needn't be directly related to BDSM - it basically just means the adoption, often pretend forced, of a stereotypically stylised femme performance with cross dressing. This may or may not be part of a ritual humiliation, or sex play, or whatever.

It seems for me, that because my sometime desire to 'feminise' myself is connected to my own internalised humiliation at failing to perform masculinity correctly, this would be a type of sissy even without the sexualised element.

It also seems, from my limited encounters with online gays on this topic, that this area is still somewhat taboo even amongst some LGBT folk.