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Depends on the gym really. All three BJJ gyms I've trained at, nearly everyone trained with nearly everyone. I'm more strict with who I roll with than most people, for injury prevention. If you don't want to get choked out by a woman half your size, you can politely decline 
in my experience the two main groups of people who are more careful about who they spar with are 1) experienced people who know better than to roll with white belts (especially big white belts), and 2) smaller people who want to reduce injury risk. Never seen gender be a factor.
