What does it mean when a guy says he's "marking his territory"?

Anonymous
My boyfriend of 4 months gave me the flu, and while joking about it, he said "I was marking my territory." It raised a huge red flag for me but I'm scared I'm simply overreacting, especially since he said it in a joking manner. Still, it made me quite uncomfortable. It made me feel like he was calling me property. Any thoughts?
What does it mean when a guy says he's "marking his territory"?
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