For those people who didn't "get" why Rina had to protect Mayu and why she chose not to "come out" to her friends about their relationship, the answer is simple.
This manga is flawed
Yeah buddy, way to go by missing the entire point of what it was trying to say. I fucking dare you to come out to your school group that you're gay and have a relationship in such a conservative space as japan, you wouldn't last a day