Am I being selfish for this?

My sisters boyfriend is staying over (he stays over most nights), and the only towel that he could use was one of mine. I bought it, and me and my sister never swap or share because its a bit unhygienic, even if its washed. However, she called me selfish tonight because I wouldn't let him use one of mine. I don't understand why I should. He's not my boyfriend, and he's got his own house with his own towels that he could bring. She chucked a big hissy fit about it. I know if it was the other way round, she would not let a guy I was seeing use one of her towels. She is crazy about that sort of stuff.

So tell me, am I being selfish for this? I find it disgusting and unhygienic that someone else, who isn't even my friend or boyfriend would use something to dry his dirty butt and crotch, when I have to use it in the future.
Am I being selfish for this?
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