A lot of people get hung up on the "forced" part. Either they object to eroticizing a fantasy that involves non-consent, or they don't understand how it can possibly be "forced" if the guy being feminized is getting off on it. These people must realize that words themselves can function as sex toys. We call it "forced" simply because it turns us on to do so.
With forced fem part of the fun lies in creating the feeling that it is forced. There's an art to doing only those things which a submissive has clear-headedly agreed to, and still making him feel like he's out of control, swept away by the irresistible force of his Mistress's will and desire. The materials with which we erotic artists create such experiences are our words, along with props like costumes and BDSM gear. For a forced fem scene I might put a pretty dress on a guy along with panties and a pair of breast-forms... bind his hands behind his back... make him kneel in front of me, hold a dildo up to his face and order him to suck. For the boy whose mind is kinked just right these actions alone are powerfully arousing. Add my detailed instructions on how to suck a real cock and descriptions of what his life is going to be like from now on as one of my sissy sluts and I'll have him turned into a puddle of subby goo. He'll be convinced on the visceral level of his throbbing erection that he's been sold into slavery and is about to be turned into a mindless bimbo fuck-toy.
Why a guy would want to be transformed into a girl is another question. Femininity in our culture represents many things to many people: being clean, polite and pretty instead of dirty and rude, being free to have feelings, being allowed to enjoy sensual pleasures like silky lingerie and fragrant bubble baths. But perhaps the strongest and most common urge expressed by a feminization fetish is a longing to be desired. The feminine is the pursued, the sought-after, the lusted-for, the wanted. And lots of subguys, even those who aren't into forced femme, share this desire to be the one who is chased.
A feminization fetish also very frequently goes hand in hand with male chastity and cuckolding, and is usually served with a heaping side order of erotic embarrassment. But I'll save discussing those fetishes for another day.
