I'm wondering if those who live with the 'double stigma' of polyamory and something else not mainstream, find it harder to come out?
Yes. Because not everyone is safe. And it doesn't even need to be "double stigma." Just dealing with one thing could be hard enough.
They might not be safely employed. Like if they come out about whatever it is, they could lose their job or be stuck in low level positions, never promoted.
They might not be safely housed. Like if they come out about whatever it is, landlord, neighbors, whoever... will make living there in that area hell. It might not be "big" stuff. But a long series of "smaller" mean. Like "not getting" phone calls for making repairs and making you wait longer before a plumbing issue finally gets fixed. And when it gets fixed, fixing it, but not really cleaning up the mess from fixing it. Or doing a half assed job because they know you won't complain too much because you don't want to make waves.
They might not be safe in person. Like if they come out about whatever it is, they may become subject to hate crimes/attacks.
They might not be safe with their children. Like if they come out, some relative or some other person will try to sue for custody or make problems for them with Dept. of Children and Families reporting made up crap.
Just because something is legal, doesn't mean it is culturally accepted. That's much slower to change.
Because people sometimes have to live where they live, and don't have the money/ability to move elsewhere, they have to adapt to their conditions.
Sometimes that adapting means "passing."