It's not like Dean doesn't have a chance to have a long-term heterosexual relationship. Even meta-universely, they tried. The writers tried, the studio tried, the executives tried. They specifically asked to have a female lead in the show and have something with Dean. So they inserted Bela, putting her name in the lead cast for a season, preparing to make her story longer. She's cute, annoying, the whole enemy to friend to lover curve right there, waiting to unfold. She's beautiful, chatty, charming, witty, exactly the type (they think) Dean would like, like the kind of waitress he'd usually take home in early seasons. They let them be pretended couples in one episode, let Dean call her "my wife", dance in fancy dress and suits, let Bela flirt and explicitly say the words they'll make so much love after that ("when this is over, we should really have angry sex.") and make Dean blush. And even make their journeys parallel lines: Dean has the demon deal and is going to Hell, so does Bela, they can relate so easily. And if it really works, they can make them reunite in Hell and fight out, or get brought back together by angels and keep on. But no, it doesn't work. It completely doesn't work. And it's Dean's choice. The chemistry just isn't right, Dean isn't interested, even God of their world (writers, the studio) can't make them work, so they kill off Bela in the end.
They then tried Anna. Who said Dean could be angelsexual? They say, okay let's see if he can get together with this cute pretty little angel. Once again, she's the type that (they think) Dean would fall in love with. They paved the road for Dean again, meta-universely. They worry Dean would find it unacceptable to date a nonhuman inhabiting a human body, something too similar to the monsters heโs hunting, which might end up becoming the obstacle that prevents them together, so they thought about it carefully and fixed it way earlier than they even met, giving Anna a humanly natural-born body. Of all the angels, Anna is the only one whose body is her own, not taken, not possessed, not a vessel, but a real body. The only one (they think) Dean has an actual chance to fall in love and be together with. They make them have the "last night on Earth" talk and actually try the sweet make love scene. But once again, it doesn't work. The forced sex tension doesn't work, the forced mutual attraction doesn't work; there is just not enough chemistry. So they had to give up Anna, just like they gave up Bela, killing her off in the end.
We're not even starting to talk about Lisa and the one-year living-together and the kid that very much likely is Dean's own son (if they choose to settle with that storyline).
And then there is Castiel, who they totally didn't plan anything, didn't think anything would happen. Cas should just functionally smooth the plot problem, bring Dean back, and that's it. But Dean decides no, he'll hang on tight with this angel; he doesn't mind he's in a male body; he doesn't mind he's possessing a poor vessel, not having his own body; he doesn't mind he isn't even human; he doesn't mind this angel is everything no one think he'd be romantically or sexually interested, he's nerdy, dorky, unaware of any pop culture, doesn't flirt back, so not his usual type. But it just works. And Cas decides he'll choose Dean too, helping him out every chance he can, sacrificing himself for Dean every year. They try to take Cas away from Dean, meta-universely kill Cas off, multiple times. But this time, this is the thing that doesn't work. Dean can't move on. Every day when Cas isn't there, Dean is miserable, pained, aching, ready to kill himself. They can't move the story forward without Cas coming back, so they just have to bring Cas back, and Dean is content again and able to continue his life.
Who said fictional characters don't get to have free will to decide their own fate?
God (Writer/studio): Bela will be your partner. You'll like her. The two of you can have as much sex as you want.
Dean: No.
God: Anna will be your partner. She's an angel, and the two of you already had sex and you can have so much more.
Dean: No.
God: This is Cas, don't mind him, he's just here for like a minute and you'll never see him again.
Dean: I'm keeping him. We love each other. I'll spend the rest of my life with him.
Destiel is free will, is their own choice. It's true love's power. Dean chooses who to love all by himself. So does Cas.
