A Rose Is A Rose Is A Rose
1. Draco
Malfoys don't believe in anything as plebian as love.
They have arrangements.
If a Malfoy were to ever fall in love, they would most likely mistake the emotion for something akin to dysentery and take the required potions to rid themselves of the sensation. Or. Depending on the situation, they might simply remove the offending person from their sphere of associates. And the plane of wizard existence.
No, love is not to be born, and 'arrangements' make life far more palatable for all parties involved. This is why Draco has an arrangement' with Weasley. They don't fancy each other; they don't even like each other. 'Fundamental Loathing and Disgust Channeled into Brutal Shagging and Extreme Violence' would be a more appropriate title for the relationship that exists between them.
However.
There is a relationship. There is something.
Draco and Weasley have an Arrangement.
And this is why Draco takes it so poorly when Weasley throws him over for Longbottom.
Of all the possibilities in their entire sixth year, the only replacement that Draco would've found more offensive to his sensibilities would've been Potter. Or the mudblood. But since Draco knows for a fact that the Weasel doesn't swing that way, this Weasel/Longbottom pairing will have to take the brunt of his wrath.
Unless he decides to make a new arrangement of his own.
2. Ron
Ron never gave his sexuality much thought, because he always fancied Hermione. Except for when he fancied Harry. But they were his mates - his best mates - and he always reckoned that fancying them had less to do with their body parts and more to do with how much time they all spent together. It's rather clear that Ron fancies people he's comfortable around, and the only person who he's more comfortable around than Harry and Hermione is Neville. Except that Ron never thought he would fancy Neville, because Neville used to have funny teeth and he couldn't play Quidditch to save his life. Neville was the most inept wizard Ron had ever met. But then they all went to the Ministry and there were flying brains and broken noses and -- and it turned out that Neville wasn't quite the mess that Ron had always thought he was.
And then sixth year started, and it turned out that Ron was attracted to blokes with scars and good lineage and very broad shoulders. Harry. Neville. Draco Malfoy.
But Ron only shagged Malfoy a handful of times, and only because he knew that Harry would never give him a leg over, and Neville. Well, Ron genuinely fancied Neville -- he didn't want to taint that with something as dirty as what he did in dusty corridors with Malfoy.
But then Neville came to him.
3. Neville
Neville used to fancy Harry Potter something rotten. Not just in that stuttering, clumsy-with-everything-he-touched-way, but in that obsessive, notice-lint-and-stubble-way. Neville used to know Harry's schedule inside and outside and every side, including the shortcut Harry took for Tuesday morning early practise. Neville can be very attentive when he wants to be. Of course being attentive to Harry also meant being attentive to Hermione and Ron and everything they did with regards to Harry.
Neville saw Ron watching Harry long before he saw Ron watching Malfoy.
Neville also saw Malfoy watching Ron back.
It took Neville a while to realise that Ron was also watching him. He blames it on Ron's fringe, which tends to obscure his eyes. It's hard to realise someone fancies you when all you can see is how pink their mouth is and the way their tie is loose at the collar. Never mind the whole bit with the rolled-up sleeves and Ron's really pale forearms and long fingers.
Neville could've gone on staring forever. There was nothing to stop the cycle.
But then he saw the greenish-yellow bruise high on Ron's forearm.
It was the exact same shade as the bruise on Malfoy's wrist.
4. Harry
Harry's idea of love is skewered by his family, his quasi-family and entirely too many Valentines on the 14th of February. These days it's hard for him to know who loves him for him and who loves him for his surname, so he prefers to err on the side of not getting his heart mashed up in the process, and if that means he's alone more often than not? Well, he's not terribly sure how long he's going to live at any rate. Not that that's really relevant since everyone he loves seems to leave anyway.
Harry misses Sirius every day, as much, if not more, than he misses his mum and dad. He knows he loved Sirius. He knows that Sirius loved him -- even if he never said as much; and Harry knows everyone he loves is going to die, which is what makes everything so much harder right now.
He loves his mates and he loves his ramshackle family, but the love Harry feels for Hermione and Remus and the entire Weasley clan is not the same as the love he feels for Ron.
To put a pretty fine point on it - Harry fancies Ron, and this is why he finds himself picking fights with Ron for no reason. Not because he wants to, but because on some level it's easier if there's a distance between them that won't make the inevitable loss so hard. Except it's a lot harder than Harry reckoned it would be.
Harry thought he could deal with his assumption (wrong) that Ron would never fancy boys.
Harry thought he could deal with Ron and Neville being together.
He thought he could deal with the idea that after he's dead, they'll most likely live a long, happy life together.
What Harry can't cope with is hearing from Malfoy what it's like to have Ron's dick up his arse or what Ron tastes like on Malfoy's tongue; and all the anger over what Harry's never going to have makes it much easier for him to do something very stupid indeed.
Malfoy calls it an 'arrangement', but Harry's just making sure he won't fall in love again.