I've always been unsure if they feel they have to hide it, or if they're just feeling private, or if it's something to do with Nanoha's ninja upbringing — but the fact that everyone keeps being surprised at Vivio having two mothers doesn't speak well for the heteronormativity of Midchildan society.
(I also wonder sometimes how skewed a view of Mid and the TSAB we all have from being introduced to them by a child archeologist and a very particular portion of the Navy.)
