It's actually very simple. Sissification is a process of humiliation via feminization. But I don't feel humiliated by my own feminization - I feel empowered by it! Girls are not, to me, inferior beings; they are better than men in almost every way!
Now, I understand the sexual appeal of humiliation. And I don't want to rain on anyone's sexual fantasies (believe me, that's the last thing I want to do) - and as long as you understand the difference between those fantasies and reality (i.e., not humiliating real females outside the bedroom), then play on.
But it does seem kind of unfortunate, the sexist implications of sissification - and I think it would be nice if we could accomplish the goal of sexual humiliation in a less gender-rigid way. So, like, instead of identifying everything feminine as something humiliating, you could focus on the submissive aspect, and various roles (like servant, housekeeper) and outfits (something unglamorous like rags, or embarrassing like a clown costume) that don't carry a direct gender-specific connotation.
That'd be nice, but I do understand that you can't just reason your sexual desires to be whatever you want, and that, especially, non-PC roles can be extremely thrilling in large part because of how 'bad' or 'wrong' they are.
But, if you've ever wondered why I, personally, don't identify as a sissy, it's because I don't find feminization humiliating. It turns me on, to be sure, but for a wholly different reason: that is, I'm attracted to femininity itself, and the idea of being feminine; humiliation doesn't come into the picture at all.