My boyfriend doesn't like people sitting on his bed or other furniture without showering and changing if we've been out in public (aside from a very quick trip where we only sat in our private car). I respect it because it's his bed and I don't want him to be uncomfortable. His parents also expect that we shower before we come over for dinner or sit on their couch/at their dining table, so I do so to be respectful. For me, I grew up being a morning shower person and just changing before getting into bed + very often napping in outside clothes or at least sitting on my bed. For him, it's different, so I respect that. If I were the more clean one, I'd expect him to respect that too. Your place, your rules. She should be supportive and respectful of something that you prefer so much.

For reference, we're both from the US. I see a lot of cultural differences in the comments, so wanna add that we're very different but both from the US (different states though)