If you replace "online games" with "flag football", would all the above apply? Because, at my work place, we have a co-ed flag football league that plays on weekends and causes me to miss time with my spouse, make both male and female friends, etc. Other than the time of day and the lack of exercise, I don't see why this is any different. Moreover, this is extremely unlikely to ever lead to cheating because the female gamer is probably across the country.
As someone who has played online games before, I can tell you it is more fun with other people, same as it is more fun to go out with other people.