Sado-Masochism on the Trains of Singapore

I really ought to be working on my term paper, but boredom set in and here I am, about to recount some weird thing I saw on the train some time last month - one of the side effects of living in an urban, claustrophobic country: so there I was, on my way home and minding about my own business like everybody does. But I was standing close to this couple, standing at the glass panels near the doors, and I couldn't help but indulge in my voyeuristic tendencies in the episode that ensued - they were practically inviting me to do so.

But it's not as if I was listening in right from the beginning, so when I started to tune in, the girl was already in the thick of a plot trying to do-in some person she didn't like staying together with her - a sister? A cousin? A roommate?

(in Mandarin, loosely translated from memory):
"Hey, help me think of something to frame her (I'm assuming it's a her, since Mandarin doesn't have a gender noun in the spoken form, ya) lei" (Spoken in a sweet voice belieing her apparant taste for schadenfreude, as it later turns out.)
(Her boyfriend keeps quiet, as far as I remember.)
" I know, I'll cut up all her clothes, and then make it seem as if somebody else did it!" (Boyfriend says some disparaging remark about her stupidity or something to that effect.)
"What, can what, she'll never think it's me lor. Who else can it be? Can only be her."

And I, realising that I have been quite quietly impolite, decide to tune out. But I only manage it for a while, because for some strange reason this girl started slapping her boyfriend, as if for the fun of it. That naturally pisses him off, and he utters the requisite swear word in Hokkien. So she changes tactics, and starts flicking him in the ear instead. Now he really loses his cool, and chee bye he said, this time with more menace, holding his fingers on the ready to flick her back - "Do you want me to do it to you? Huh?" The offending girlfriend finally desists and turns on her womanly wiles, acting all coy and demure... and before I knew it the two were back in lovey-dovey mode again.

At that point I was thinking: Gee, if they're like that in a public crowded cabin of a train, goodness what they do in the privacy of a bedroom. Bondage and whips? Seriously, that rates as one of the weirdest couples-I-am-unaquainted-with. Couples are always mysterious and uninterpretable entities in the first place, but this pair.... gosh.