I see what you mean, but for these examples, I'd struggle to describe why the terms "convention" or "idea" aren't more accurate.
Borders are things that we can observe and agree the position of, and change by agreement. They're a different category of thing from the ineffable characteristic of "gender", which is supposedly a "social construct" yet can't be observed by anyone but the person claiming to have one.
The idea is that anything we describe using language is a social construct, because of some intensely navel gazing consensus reality philosophy.
It's like "if a tree falls in a forest, but there's nobody around to hear it, does it make a sound?". Obviously it does, but it's a philosophical point that while a tree falling causes vibrations in the air, it must be perceived by someone or something to be sound (although there's a more batshit philosophy of "if there's nothing to perceive anything, we don't know that tree even exists" but that's not what they're getting at). The way reproduction works doesn't change if there's no people to categorise it, but it's not categorised if there's no people. And because systems of categorisation are invented by people, whether we say humans are being two discrete sexes that can have things wrong with them
or that sex is a variety of traits that are bimodally distributed in a population are both products of a human mind observing natural phenomena and inventing classifications for it.
Colours, shapes, species, sex - they're all social constructs in this framework. Is a round blue pebble or a female tabby cat innately those things or just how we're experiencing them? Well yeah obviously they are those things, even if some people across time and place would have considered it a "smooth green rock" or "yellow furry animal that had produced a litter of more yellow furry things" because that's what their language had available. Unfortunately a lot of people wheeling out this argument are only getting it fifth-hand filtered through twitter and so think declaring "x is a social construct" means "x is completely wrong, and therefore y is correct" without realising that x and y are equally valid and invalid in that viewpoint, because "sex is a spectrum" or "sex and gender are different" or "there are more than two genders" are all
also social constructs and merely saying one thing is a social construct doesn't demonstrate why your social construct is the one people should observe.
It's also a luxury of the thinking classes. "How do I know if the universe exists, when in a universe of infinite possibilities it's infinitely more likely I'm a
Boltzmann Brain that has spontaneously formed in a void with false memories and a hallucination of what I think is reality" is not a useful or meaningful thing to consider if you're late for your shift in the factory because your kid is sick.