The population of Canada is close to 50/50 with 49.5% male, 50.5% female. It hasn't led to an absence of social/sexual tensions.I'm not talking about little jaunts out into space, but imagine missions that are many years or decades long. Humans still have primal needs, and being a scientist does not make someone above reproach. Just like police officers aren't "supposed" to commit crimes, namely assault or rape, but it happens all the time. I just think it's simply a better and safer environment if the male to female ratio is equal, or close to equal to avoid social and sexual tensions.
Besides which, assuming having a 50/50 split on a space mission would dissolve such tensions assumes everyone is willing and able to satisfy such requirements. I think that is morally wrong in and of itself!
NASA is a professional organization, unlike the police forces (unfortunately), and cosmonauts undergo rigorous entry examination.