Contrast with instruments that don't use the harmonic series and are always in equal temperament, like woodwinds and keyboards. Also contrast with fretless stringed instruments, for which there is no inherent temperament whatsoever if you avoid open strings, since the tuning of each note occurs by normal fingering, which I suppose is considered natural.
[0] https://www.reddit.com/r/trumpet/comments/1zy0w8/trumpet_phy...
PS woodwinds aren't "always in equal temperament" either, the player has to adjust for tuning compromises in these instruments as well. Not that being in equal temperament is even something an orchestra aims for.
Interesting about woodwinds not being equally tempered! I see another commenter points out the same, so I take that back as an incorrect assumption. So I wonder why they wouldn't build them as such, given that it would reduce the worst-case required bending to achieve arbitrary-key just tuning. I think the octave key is typically the only critical use of the harmonic series (altissimo aside), and that's a perfect 1:2 in all systems so it's moot.
There may also be tuning adjustment available with the amount of cover for a hole, depending on the instrument.
Alternatively you can give them additional valves which are tuned slightly differently. Which is usually found on Tubas and other deeper brass instruments.
There are more technical solutions like compensator valves too.