In general you seem like you answered your own question but don't realize you did. Could flip the order of your sentences. (Edits mine.)
- I understand that some times the relationships some one might be involved can be complicated to explain.
- I don't think one must explain their relationships to any degree other than that which they choose to.
- (I see that there could be) fear stemming from the fact that you do not know how they may react to this information. (Cannot control other people's behavior.)
- I just don't understand why it seems like many people at least on these forums make some kind of effort to hide (not explain) their multiple relationships.
I think you seem to get WHY. But maybe you mean the HOW of it. Like... what does telling a LIE do for them? You don't have to tell the world everything but why do lies of omission?
In that sense you are right. Don't have to lie. Could just tell inquiring person something like "Thank you for your concern. In this case you do not need to concern yourself for my sake. This topic is not up for discussion" or "Thanks for your concern but I'm not up for talking about that right now. How about we talk about ____?" Tactfully change the subject.
IME, I find polyshipping easier at the bookends of life -- young adulthood for instance. Or older adulthood. This bit in the middle with parenting is hard enough to me without adding polyshipping relationships to the mix. If they weren't there already before kids came along, I'm not up for building them once kids are here. So... I wait.
Easier to me to wait when kids grow up and out and more of my time is free of parenting obligations.
But I can see why some parents with children who polyship might be reticent to reveal. It is risk.
Other disapproving relatives could try to take custody of the kids because they think polyshipping is bad somehow and create merry hell for the family in question. Or some other hooha thing -- people taking it out on the kids, losing job security, hate crimes (ranging from vandalism of your property to physical assault for being different) -- the world and people in it is not always lovely.
If all parties involved are on board with WHY they choose to fly under the radar and they are happy to do so, that's their business. *shrug*