It is my impression that "... and all cultural constructs are equally valid" is at least connotatively associated with the notion of a "cultural construct".
Can you say more about where this impression comes from?
I would agree with "...and cultural constructs do not represent a uniquely valid objective truth," and various things along those lines. But "all cultural constructs are equally valid" seems significantly overstating the case.
For example... I expect that most people who talk about cultural constructs at all would agree that chattel slavery and abolitionism are both cultural constructs. I doubt they would agree that they are equally valid for any understanding of "valid" that is at all relevant to this discussion.
Do you expect something different?
I think that the expression "cultural construct" implies that the construct in question is a representation not of physical reality, but of something inside people's heads.
Usually this is held to mean that cultural constructs are somewhat arbitrary, highly malleable, and do not involve laws of nature.
I'm afraid I haven't properly designed the Muggles Studies course I introduced at my local Harry Potter fan club. Last Sunday we finally had our second class (after wasted months of insistence and delays), and I introduced some very basic descriptions of common biases, while of course emphasizing the need to detect them in ourselves before trying to detect them in other people. At some point, which I didn't completely notice, the discussion changed from an explanation of the attribution bias into a series of multicultural examples in favor of moral relativity. I honestly don't know how that happened, but as more and more attendants voiced their comments, I started to fear someone would irreversibly damage the lessons I was trying to teach. They basically stopped short of calling the scientific method a cultural construct, at which point I'm sure I would have snapped. I don't know what to make of this. Some part of me tries to encourage me and make me put more effort into showing these people the need for more reductionism in their worldview, but another part of me just wants to give them up as hopeless postmodernists. What should I do?