Semen is fluid released from the urethra during sexual stimulation that contains both fluids produced by the male reproductive organs as well as sperm.
Contact between one's own semen and healing body modifications, while not typically dangerous, may be uncomfortable and can irritate the healing in some individuals. Contact with someone else's semen can put one at additional risk of STDs because the healing modification is effectively an open wound. It should go without saying that protection (ie. a condom or other barrier) should be used especially when fresh genital piercings or other healing modification are involved to minimize exposure to foreign bacteria.