^ Most just prefer LukaxMiku. There is no such thing as canon in the world of Vocaloid so ship away to your heart's content.
Coming from the Negitoro fanhood I don't particularly care for either relationship over the other. Everyone is reading way too much into this and projecting what they, ironically, want Rin to be rather than what she was. Being the aggressor does not mean you are aggressive. Pushing for what you want does not mean you are pushing the other person down. I don't care for the concept of romance budding purely from sex but nonetheless Rin played a semi role of the healthy sort of yandere here. She protected Miku in a possessive yet open way. She wasn't hoarding Miku but she was throwing the proverbial blanket between her and Len.
If you see Len as a serious candidate for a relationship here you are wayyyyy off. He refused to listen to Miku when she implored him in a defenseless state, and instead of talking to her and apologizing or trying to fix things, he let her fade away. And when he learned that she was practicing under the conscious pretense for him essentially, he threw her aside.
Rin constantly asked Miku to let her know if it became painful. She may have been the "top" but she listened to Miku and only did what she asked of her. And after it all, she was honest. She confessed and presented why she was doing what she did. But she also makes it clear she won't give her up to anyone else anymore...but it feels more like Rin proclaiming her resolve than it seems a possessive unhealthy aggression. We don't get much insight into Miku, but it is certainly clear that Miku wants a relationship with Rin, as clued in by the end scene and how they hung out all the time during the day (in the photos) and seemed uber happy.
I don't care for this work too much - again, from the Negitoro fanhood - but it was nice taking an objective look at fairly complex yet simplistic in design love triangle. I actually like the triangle, and I freaking hate love triangles.