Some people, specifically the critics, think Merida is gay based on the stereotypes they have of lesbians. Clearly since she didn’t fall in love with a man like every other Disney princess, she is a lesbian. That is exactly how they think.
However, I do think some people see Merida as gay because they identify with her and they are gay. I think people see Merida as asexual because they identify with her and are asexual. Other people see her as straight because they identify with her and are straight. People who identify with her put their identity on her because they relate with her. The writers left her sexuality open. No one said “Merida is straight.” Her sexuality didn’t even come up when creating the movie. People are going to project their identity on characters they relate to and that is perfectly okay. There is no reason why Merida cannot be a lesbian, bisexual, or asexual. There is no reason why Merida cannot be straight. At the end of the movie, she was single. Assuming there is never a sequel to Brave, Merida can be whoever the hell we want her to be when she grows up. She could be a lesbian princess or a single Queen. It’s up to how we see her.