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It can be sexy, especially if it's a guy having sex with a girl that's fully clothed and pushing her panties aside. It's like they're in such a hurry to fuck that they don't bother undressing first. There's at least one artist that absolutely loves the idea. For example: post #9219278, post #7220738 and one with a fun expression post #7430063. Even though their art's low resolution they end up with hundreds of up-votes on Gelbooru. There's a some great expressions on the various girls faces as well, but be warned that a few are rape. Most of them aren't and there's over 1,100 of their images here.
Being partially clothed can be more sexy than being fully clothed, even if the person's functionally nude. (Like a girl with panties off, skirt lifted to expose her pussy and shirt lifted to expose her breasts.) I think it's because it adds some extra naughtiness to the sex. Taking the time to get fully undressed before having sex is normal and boring, doing it partially clothed isn't.
It's similar to exhibitionism in a way. Exhibitionism is sexy because it's nudity in places you don't expect it. Exposing my pussy at home is no big deal, but if I'm doing it in a public park it's a different story.
Clothed sex, in artistic perspective, is quite fun. You can add multiple layers of expression through clothes. Wanna determine age/personality/scene intensity? Add clothes. Me, personally, don´t use much clothing but love jewelry.