As a man, I'll say I absolutely agree. I went to an all-boys high school, and teenage boys simply do better without girls around. Boys are more likely to engage in not traditionally masculine activities when girls are not around because there is no need to impress. Furthermore, girls often mature faster than boys, and this leads to unnecessary stress for both girls and boys at this stage. Furthermore, a lot of what drives teenage boys is competing against other boys for attention from girls. This dynamic is messed up if there are girls around. This damages girls too. Girls often complain that boys are uninterested in competing with them. This is because beating a girl does nothing for boys. They want to compete with boys. If each gender has their space, they both get to flourish.
I don't think single sex environments harm future gender relations either. If anything, they make it better. Both genders need friends of the same gender uncomplicated by romantic feelings. Furthermore, from my own experience, the boys at my high school, and the girls at our sister school, found no trouble getting married, having kids, etc.
Speaking of our sister school, the women they produced were independent minded, competent, competitive, and had leadership experience. Without the need to compete with boys, the girls developed their own leadership and competitive structures that they excelled at.