As many Chronicle readers, I have coached numerous youth sports and, delightfully, was able to watch my children compete in many more. My experience is, that in most sport, the benefit such scrimage by opposing teams of the opposite sex diminishes at around age 13. After that it appears, and often is, likely to be dangerous.
If athletes can coordinate their social skills and courtesies into their competition, there are many sports, both major and minor, which will accommodate "mixed company" might participation. Hard training for important meets, matches, and games does not seem to offer that possibility.
There are of course some exceptions..FORE!
It's really about trying to simulate more rigorous competition beyond what your scout team can offer. In the case of teams like the University of Connecticut and others, it is important to provide the women's basketball team (as a revenue sport) with such practice players from the population of male intramural athletes. It would not be an issue if the UConn team did not have a male coach (who was just named to the Hall of Fame(s)) and it's just regulatory sour grapes on the part of those who refuse to innovate. It is even more sad that some of the loudest voices against such issues are women who don't quite have the same record of success. Danger seems quite irrelevant.
BTW, speaking of fair play, golf does raise the issue of Michelle Wie playing in PGA events...