There is indeed a player in this group who wants to dictate loot distribution.
It's you. You are the loot hog and this player's actions are the result of you being unwilling to compromise fairly.
As already described in Dan B's answer, the normal way to distribute loot is to decide which character can best use it, and give it to them. It doesn't matter which character opened the chest, if they can't use the sword they should give it to someone who can - even if they 'plan to learn proficiency later', as you say.
If your justification for this is that your character would not want to give up the powerful sword to any other member of the party, that is also a problem. It reeks of "My Guy" syndrome, and you should carefully consider whether or not this is right of you to do.
A few things to clarify regarding this situation - it was not right of the other player to decide, for themselves, that they would steal the sword from you. You are right to be upset at this - but you also need to consider why they would do such a thing, and the fact is, you were being unfair to them too.
Not to mention - using a magical weapon like this requires magical attunement in 5e, so being a 'sword distributor' doesn't even make sense in terms of pure game mechanics.
In short - no matter your reason for wanting the sword, this is a poor idea and will not work out in the long run. You should open discussion with your group about who should have the sword, and understand that there will always be chances for you to find more magical weapons.
Remember - at a certain level of wealth, you can ask the DM if you can just buy a magical sword.