Make them clip their fingernails!!! Or wear latex(or non-latex) gloves, they have a great sensation. If you are having sex with a lady with long nails have her put cotton balls on her nails inside the rubber gloves, it helps them live longer and dulls the edges. Also make sure you are good n wet, a little lube never hurt anybody, but dry skin + friction = blood
Happens to me, never really thought or asked myself if its harmful but hasn't caused me anything! just lasts a few hours( pinkish blood spots) and then its gone!