This is about Sebastian

Sebastian is not gay canonly or officially

His sexaulity was NEVER mentioned or confirmed to be gay in the show. It was never officially canon and was never spoken about in the show at all.

And when talking about his past relationship to Klaine he says THEY and THEM. Not he.

He says: it never worked out with my relationship with them as they...

Nowhere in the show says he's gay. Just because he flirts with Blaine, tries to interfere with Klaine and goes to a gay bar it doesn't make him gay otherwise Quinn's one night stand with Santana makes her a lesbian but it doesn't at all.

Grant doesn't even confirm or mentions Sebastian as gay. He says:

I originally auditioned to be a tap dancer in the background for the performance of Anything Goes and got the call I was auditioning for Sebastian.

He also says that he hopes Sebastian is Sue's heir.

Nowhere does Grant say Sebastian is gay

His gay sexaulity is not officially canon unlike
Santana
Kurt
Adam
Blaine
Brittany
Dani
Jeremiah
Karofsky
Unique
Sheldon
Walter (the old guy that Kurt dates)
Eli (Blaine's one night stand)

Whose sexaulities are canon and is harmful to erase.

Just because it's on Wikia doesn't mean it was actually confirmed. Anyone can write that based on assumptions towards Klaine because God forbid any guy who even talks to at least one of them is gay and is after them...

As like according to Klaine and the klaine fans no two guys can actually have friends who are guys because it means something and they were flirting and trying to get into in-between Klaine and is after Blaine.

When quite literally it was only Kurt to assume that Sebastian was after Blaine and was interfering and Sebastian to irritate Kurt like he loved to do all the time probably went along with it to see him squirm since he hated Kurt a lot and would do anything to knock his ego down.

So shipping Sebastian with Rachel or even Marley (like some do) is okay to do because it isn't harmful and it isn't erasing anything canon to do with Sebastian at all, just like Shipping him with guys is okay to do as well.

Whereas

Shipping him with Santana and Dani is harmful because it's erasing Santana's and Dani's canon sexaulity of being lesbians, and its dismissing them.