Are there any queer Space Marine Legion in Warhammer 30k or 40k?

You also have the Emperor’s Children.

Games Workshop used these originally-queer “transgressive” motifs for Slaanesh and the Emperor’s Children. Without ever explicitly saying so, EC background coyly alluded to them being sexually transgressive in a “you do the math” sort of way. They were very up front about the noise/music aspect, the sadism, masochism and narcotics aspect… but the “other forbidden pleasures” that the Emperor’s Children indulged in were always left unspoken with a wink and a shrug.

Back in the late 80’s when 40k was born, gay/queer/trans was still widely and popularly seen in most Western societies as “transgressive.” A quick and easy way for other scenes to clearly convey their own transgressiveness was to ape and caricature the fashion associated with certain gay and BDSM cultures: tight studded leathers, spikes, straps, chains, dog collars, that kind of thing. That is why punk and heavy metal used aspects of that look.

What are "demonic attacks" and how can one tell when they're happening to them, or someone else? How would one go about dealing with it?

The process of turning a pre-pubescent boy into a Space Marine essentially turns them into biological war robots.

…Games Workshop has laced subtle connections and allusions (some of them very dated and derogatory) to LGBTQ culture into Space Marine lore.

That said…

Why do people keep saying they have evidence and have presented it that proves you're wrong even though they have none and haven't presented anything? Furthermore, what do they think you're wrong about?

Space Marine background lore is consistently clear that Astartes are asexual - they neither reproduce sexually, nor do they feel a sex drive. The lore is deliberately unclear whether they even still have a penis, and there is no reason to think that they retain their useless testes. In one of the novels, an Eldar refers to them as “geld warriors.”

TL/DR: No, there are officially no queer Space Marine Legions. There are also no heterosexual Space Marine legions.

Of course, you have the Dark Angels and Lion el’Johnson, named directly for the poem about hiding secret homosexuality by the poet Lionel Johnson. The First Legion is not secretly queer - their secret is The Fallen - but the original connection is what it is.

Is it true that people who are possessed by demons cannot see them until the demon is cast out? What is the reason for this?