1. Focused on the girls, with the guy being covered up, out-of-frame, or similar. The girls should be emotionally focused on each other almost completely, perhaps going as far as treating the guy as little more than a sex toy used to give each other physical pleasure.

2. The male in question is androgynous, or outright feminine, enough to be easily mistaken for a girl (e.g. post #371659).

3. Very feminine futanari, easily mistaken for a girl (e.g. post #170462).

I get why someone would include this in the pool, it can easily be mistaken for yuri save for the fact that it's just one woman, but ingenious masturbation doesn't fit under the criteria listed for the pool.