How so? If she had used a female sub, then it wouldn't be an allegory--the message would be mixed. The audience would be saying, "so does this society have these expectations of her because she's a sub, or because she's a woman?"

In our society, we are so inured to seeing women go through the sorts of indignities that John goes through in this story that often as not, they don't affect us anymore. By having these experiences happen to a male character, we are confronted anew with how arbitrary and insane it is to place such expectations on someone based solely on their gender, or how they fuck, or anything else. I don't think the story at all supports the notion that, in BDSM, submissives are always women/feminine/whatever; however, it does point out that, in "mainstream" traditional relationships, men are expected to be dominant and women submissive, and the ways that men and women are treated because of those expectations.