Same as other comments, I think is a psychological issue, but not related to being gay, is more about the shame of getting hard while being observed by a person he loves, his past partners or his family, considering the weird time with his aunt And this explains why he got hard with ML, because MC always thought that all that sex was for educational purposes, but when he notices that he likes ML, he couldn't get up And also the stress and anxiety of being point out as if something was wrong with him (getting hard while being kid or being gay)
Same as other comments, I think is a psychological issue, but not related to being gay, is more about the shame of getting hard while being observed by a person he loves, his past partners or his family, considering the weird time with his aunt
And this explains why he got hard with ML, because MC always thought that all that sex was for educational purposes, but when he notices that he likes ML, he couldn't get up
And also the stress and anxiety of being point out as if something was wrong with him (getting hard while being kid or being gay)
That's my input