How do I say I'm in a situationship?

So I'm currently in a situationship, but I would still technically consider myself single. Even though I'm technically single, I have no desire to talk to or see other people.

How would I say this if someone asked me out? Would I say I'm unavailable? I feel like that makes me sound rude. I don't want to say I'm taken, bc it's not true, but I'm not going to say I'm single either because I don't want them getting the wrong idea.
How do I say I'm in a situationship?
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