Almost any space of value is a hierarchy, explicit or otherwise. Women automatically and necessarily invalidate male hierarchies simply by participating in them. Because competing directly with women is lose/lose for men. When you win - and men almost always win on fair competition - then you will be accused of having some nebulous unfair advantage. You will then be handicapped. If you lose, then you’re a losing loser who deserved to lose - even after you were literally handicapped for no reason other than your gender. Men recognize this dynamic instinctively. We have evolved to avoid and abandon co-ed spaces.