There are a few characters who are canonically straight who, when I see people make them romantically involved with someone of the same sex, I go "ew". I think of those characters as 0 on the Kinsey scale, and it grosses me out when people force their sexuality into something else. I have experience with people trying to do that to me (emotionally and through arguments), and it sucks.

HOWEVER I have a similar reaction when people force canonically gay or lesbian characters who read as 6 to me into het relationships. I feel that's even grosser because of straight privilege.

I think people should have to bring the same amount of "proof" to the table about a character's sexuality no matter what the character's sexuality happens to be.