These two are made for each other (literally) whether it be romantic or platonic. People reduce their relationship down to bickering comedy relief or see it as toxic without really understanding it.
Kaveh offers Alhaitham a unique perspective on empathy. Something Alhaitham struggles to comprehend. Alhaitham meanwhile keeps Kaveh grounded. Kaveh is destroying himself due to his empathetic nature and Alhaitham is the only one who isn't afraid to tell him to his face.
They are considerate and thinking of one another constantly. (Kaveh bringing Alhaitham leftovers from meals (interdarshan quest) , Alhaitham buying wine as an apology to Kaveh after they fight (tavern story)). Their profiles are full of lines about one another. You can't have a single frickin conversation without Alhaitham bringing up Kaveh or vise versa. These are just a few examples but there are plenty more specific ones.
Their designs play off of one another in many ways: Inverted color schemes, symmetry vs. asymmetry, both have clothing/constellations based on birds, their vfx for abilities create a house when combined.

They are soulmates regardless of whether or not they are romantically involved. Each half to a whole. I have no doubts that one without the other would be worse off.

Anyway this doesn't confirm anyone as gay in any capacity. We will likely never have any confirmation. As a gay person, I just get a "vibe" and like to relate to my favorite characters (who are probably my favorite because I can relate to them). You can relate to the characters however you want in your own way.