A territory becoming a state is not annexation. The US annexed Hawaii in 1898.
Puerto Rico is already part of the US.
The referendum in Crimea was run by Russia as a pretext after it had already invaded. It is extremely doubtful that it represents the actual views of the people living there.
The referendum was run while the region was being held at gunpoint by an invading force. There is no possible way to hold a legitimate referendum in those circumstances.
Either way regions are "held at gunpoint" by your definition. It's either going to be from one government or another. For example just a few years ago:
The key is whether the referendum actually got held and there was a large enough turnout that it was meaningful. No one was stopping more observers from coming and overseeing it, including whether coersion was happening. And anyway, what difference would guns make in how people voted? Because I don't see how the soldiers could possibly know the way people voted in a closed booth. So even if there were people with guns out there, that by itself wouldn't affect how people vote. The most they could do is try to force people to the voting booth, as they do in Australia for example, or try to keep them from showing up, as they do in Southern US states.
And besides, when it comes to Russia or Belarus, it doesn't matter that the legitimate government is dominant in the region instead of "an invading force", you don't trust the outcome of the referendums anyway. For example, Belarus just had one a week ago, do you trust the results?
Finally, a technical point: Russia had 25,000 troops in the Crimean peninsula, which it was allowed to have according to the Partition Treaty about the Black Sea Fleet, so it wasn't an "invasion" there: https://en.wikipedia.org/wiki/Partition_Treaty_on_the_Status....
That treaty bound Russia to "respect the sovereignty of Ukraine, honor its legislation and preclude interference in the internal affairs of Ukraine" and, furthermore, Russian military personnel had to show their "military identification cards" when crossing the Ukrainian-Russian border; Russian forces could operate "beyond their deployment sites" only after "coordination with the competent agencies of Ukraine."
Sending in masked troops without insignia to overthrow Crimea's Supreme Council, install a Russian puppet government and run a farcical independence referendum is as far as you can get from following the terms of that treaty.
There is a huge difference between having your troops stationed in military bases and hitting beaches and pubs in free time, and having them guns out on the streets.
Puerto Rico is already part of the US.
The referendum in Crimea was run by Russia as a pretext after it had already invaded. It is extremely doubtful that it represents the actual views of the people living there.