really

S&M usually comes from an agreement between two parties who were already interested in it, not manipulatively pushed onto the uke (who you can clearly tell did not expect this sort of outcome.)

No, you're talking about s&m as an erotic game. This is love story: 2 men who love one another, one cruel, one submissive. It's supposed to be real life: in reality, things rarely happen the way we expect. Although the cruel one allows guys to use and humiliate his lover, he hates them: he's jealous of them, for he loves his friend, and the latter loves him back, else he would leave. This is about passion, not about toy handcuffs and leather boots... But it's still a dominator/dominee-relationship. That's what I meant.

i don't understand the passion part

Passion is when love is really intense. That's why when somebody is tried for murdering his/her lover because the latter cheated on him or he thinks he did (Othello-wise), in Law this is called a "crime of passion". Love that's so heavy it becomes a matter of life & death.
Or did you mean the passion in this here manga? You can tell it's about passion because of the use of violent feelings like jealousy, anger, pain, self-sacrifice... that kind of thing. Love with a capital L, so to speak. Get my drift?

i see.....

Allright you, Yaoifangirl :-D
Know Othello? It's a supercool manga from 500 years ago. No yaoi, though... Boy & girl. But good.