Serine protease inhibitor that plays an essential role in male reproduction and fertility. Modulates the hydrolysis of SEMG1 by KLK3/PSA (a serine protease), provides antimicrobial protection for spermatozoa in the ejaculate coagulum, and binds SEMG1 thereby inhibiting sperm motility. {ECO:0000269|PubMed:15229136, ECO:0000269|PubMed:17644992}. This is the function of Ensembl gene identifier ENSG00000249139 (EPPIN-WFDC6, EPPIN-WFDC6 readthrough).