I've observed this for a number of years and have reached personal conclusions.
If you have an orderly, well maintained family, where everyone is well fed, decently (and in some places "warmly") clothed, the kids are getting lots of appropriate love, and are showing signs of being educated at or above their expected grade level, and finally that family discipline is not seen as inappropriate, you have no cause to worry. What do they care who all lives at your house and/or what names you call each other?
I've seen CFS PLACE foster kids and, in one case, an adoption into a PM home in a state where even ADVOCATING PM is technically a felony.
I know of another case where a single mom married into a family, and her ex got furious and took her to court to get the kids. She showed up in court with her WHOLE family, i.e. new hubby and his "first" wife. They made no secret of who and what they were. They showed that the kids were well loved and provided for, and they PREVAILED.
In another case, a man moved into a large home on a hill on the edge of town with his 4 wives and 7 children. Not long after, he had occasion to speak with a local police officer, who calmly said, "We know who you are, and what is going on up there. Couldn't care less, so long as y'all keep things calm. But y'all get rowdy and noisy and we start getting complaints of domestic disturbance, we'll treat you like anyone else. No special dispensations, either." Seemed fair enough.
My personal conclusion: Fear comes from the devil, not from God. Sinners run when no-one pursues. If you know you are not in the latter class, and that you are handling your family in such a way that it could handle SCF scrutiny without PM in the mix, then there is no true need to worry about the addition of PM to that mix.
To me, a more valid concern would be how to gracefully answer prying questions that SCF might ask regarding the PM aspect, such as how you handle sleeping arrangements and sex?
The truth is that the answers are no more their business than in a mono family. In the latter, is it of any importance whether Mom & Dad share a bed, have two separate beds, or even separate bedrooms? No. What they need to know is that you are NOT engaging in inappropriate activity, or playing inappropriate videos, etc. in front of the children. An answer as to the health of your sex life (Good, wonderful, infrequent, what's sex?) may be an appropriate indicator as to the health of the marriage, but that's about it.
Seems easy enough to adjust those to the PM setting.
*shrug* my 2 cents.