Fingernails are sharp. They can scratch and cut sensitive genital tissue. Those cuts can get ifnected, or just hurt like a biz-nitch. covering the fingers (and nails) helps prevent this.
Also consider that hands are filthy. They touch things and pick up germs like crazy. They you're going to stick those dirty fingers into a vagina, or use them to rub a penis? Ever watch how your average person washes his/her hands? let's just say they don't do an especially thorough job.
And then there's the matter of cuts and sores on your hands that can let germs from your partner's genitals into your body.
Yeah, so it's just that much better to protect yourself even during manual sex.
"Have you ever taken a moment to consider the alternate meaning of the word 'debugger?'" quoth Jack
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