It's harmful to the extent that you can cause chaffing (rubbing yourself raw), but otherwise, it's not a big deal. You're in control of both parts, so you should be able to tell if you're being too rough and back off a bit, or stop if you've gone too far.
doesn't the skin regenerate?
Of course. But you may have a sore or scab and it could take a few days to even a week or two to heal, and that's obviously less than ideal, for a number of reasons.
It's fine as long as it's not an all day occurance (chaffing could happen then). My boyfriend doesn't use lube ever and he's circumcised as well. I think it's fairly normal.
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Tmi I don't think being circumcised matters
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