If yes, could they change their sexuality with the help of magic? Or, like muggles, they cannot change who they are attracted to?
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Dumbledore, for one
To borrow a quote from The Leaky Cauldron:
Did Dumbledore, who believed in the prevailing power of love, ever fall in love himself?
My truthful answer to you… I always thought of Dumbledore as gay. [ovation.] … Dumbledore fell in love with Grindelwald, and that that added to his horror when Grindelwald showed himself to be what he was. To an extent, do we say it excused Dumbledore a little more because falling in love can blind us to an extent? But, he met someone as brilliant as he was, and rather like Bellatrix he was very drawn to this brilliant person, and horribly, terribly let down by him. Yeah, that’s how i always saw Dumbledore. In fact, recently I was in a script read through for the sixth film, and they had Dumbledore saying a line to Harry early in the script saying I knew a girl once, whose hair… [laughter]. I had to write a little note in the margin and slide it along to the scriptwriter, “Dumbledore’s gay!” [laughter] If I’d known it would make you so happy, I would have announced it years ago!
For the second half of the question... I very highly doubt it. First off... Why would you want to change your sexuality?? You are who you are, and there's nothing to be ashamed of (unless you're a psychopath or sadist or whatever). So I don't believe anyone would want to use such a spell. And we see many times that love is a powerful magic of its own. You cannot create love, as we see with the love portions. We see other examples of the power of love - such as Lily's sacrifice. Creating a spell to change someone's sexuality would go against the established canon that love is a powerful magic of its own.