There is actually a thread (yesterday) where a mum wanted to pull her DD out of a BJJ competition because she's got to fight men (because no women have entered, rather than the competition saying it's fair)
This is the only example I can think where it might be ok. Martial arts you often use your opponents strength against them. So strength isn't necessarily an advantage- the way you think and strategy is.
I do judo - as an adult in my 30's I wouldn't. I can't win. At 17 I may have been able to. Children compete mixed when there isn't enough in their catagory and girls often win.