I think the enjoyment of "forced" anything comes from that. The illusion that there isn't a choice, it's almost like humiliation in a way. "You're not a real <man/woman> you can't do/choose <Blank>" etc.
I think part of forced feminisation yes is the humiliation, but I think a big part of it is the idea of being helpless. Doing something that is totally out of your "normal" behavier. Its unconfortable, a bit scarey and yes humiliating but I think some people get a genuine turn on just from the act. Dressing as a women is something that apeals to begin with but being forced into it is an added kink.
I don't think the same rule can be added to other acts of force. Some people like to act out rape for example, consent to it but its out of controll how, when and where it happens. They may not be in the mood or something but are forced into it and obviously agression comes into that. For some just the idea of being "taken" by someone with agression and force behind it really appeals and is a massive turn on. I don't think that has to do with humiliation personally, maybe on some small level but its not the major part for the people I know who do this. Its the agression, helplessness, roughness, unexpectedness and the domination from the person doing the "rapeing".
So yeah with forced feminisation the humiliation does play a big part but I don't think it does with rape or say being tied down and helpless and being forced to cum or something along those lines. I personally have a fantasy of being asleep and Mr P forcefully having sex with me and waking me up like that, humiliation has nothing to do with it for me it is the idea of having no or little controll and it being rough and very dominating because I've had no chance to say no (not saying I want this with a total stranger though that doesn't appeal because that becomes full on rape :s).
If someone has begged for it its not really forced either is it? They have had a chance to say no. I'd think if a guy refused to beg you would get the idea he doesn't really want it. I wouldn't be able to beg for something that I didn't on some level find a turn on I get a mentel block over it. Someone could threaten or demand me to all they wanted and I wouldn't. With forced feminisation you would have the chance to refuse as well. Its very hard to make someone put clothes on if they are struggling and saying no, I'd kind of hope as well that if someone was saying no and struggling and trying not to do it that the Dominant wouldn't get a massive kick out of making them so uncomfortable to the point of it being unpleasurable. Most Dominants have to see the sub having pleasure on some level to be able to get any themselves.