Dean’s Siren being a man has been everything from a fandom joke to proof of Dean’s bisexuality/repressed homosexuality/whatever to proof of Wincest, and like everything about the show you’re free to interpret it how you want. The awful part of the episode is Sam and Dean’s big fight, though, we can pretty much all agree on that one. But honestly, the more I think about it the more heartbreaking Dean’s Siren is, for what it says about Dean.
The Siren turns into your heart’s desire, the perfect satisfaction for everything you love. Most people will have lots of traits they like, looks they desire, kinks they’re desperate to share, and the Siren makes them fall in love by fulfilling them all. Dean is Mr Ladies Man, king of the one-night stand - presumably if you asked him, he’d have a list of kinks and physical preferences a mile long. He’d be gold for a Siren, right?
Except the Siren doesn’t become a Busty Asian Beauty, or a Lisa-esque snarky family woman - it becomes not just a man, but a little brother. Dean is attracted to a lot of things, but in terms of love, Dean is so utterly damaged and jaded against and afraid of the concept that the only kind of love and devotion that he can conceive of is filial. He’s so completely codependent on his brother and his family that the siren’s best and only in for any real emotional, devotional connection is to be a brother.
That’s why it’s such a big deal when Dean denies Samuel the right to call himself their family. That’s why he’s so against the idea of John having a ‘normal’ family who isnt them (him) on the side. That’s why calling Bobby the closest thing to a father he had is such huge bond between them, and why ‘family don’t end with blood’ sums up the show so well. And that’s why it was such a big deal for him to call Cas a brother - the siren proved that brotherhood is quite literally the truest concept of love that Dean can comprehend. That’s not an epithet to be tossed out lightly; you’re on the same level as Sam, you really have to earn that with him.
And when that’s challenged and/or betrayed (Sam with Ruby/demon blood, Cas with Crowley/souls) it cuts Dean seriously deep. He lashes out. He’s an asshole. You didn’t see family like he did, you lied and you hurt him and you fucked up. But he’ll always let them back in, eventually, and that relationship will always be fixed, because to him there really is no stronger bond than family - than brothers.