<s>[INST] I m only having sex from behind because my boyfriend prefers it this way(from the ass not the vegina) and I ve heared it causes prostate or hemorrhoid cancer. It s worrying me very much. Ive heared also that the act causes cracks in the lining of the anal straight and so when ejaculation of semen inside the anus, The presence of the cracks lead to some leakage of some sperm into the bloodstream and since the sperm is considered a foreign body to the female body, the body produces antibodies to attack the intruder stranger (the spirm) and so when an intercourse occurs naturally (in the vagina) These antibodies attack sperm ejected into the vagina and cripples if causing infertility for life. This is worrying me very much! Is it true? Please advice [/INST] Hi, anal sex may increase chance of HPV infection which has potential to develop cancer. But personal hygiene, no of sexual partners also play a role. Anyway anal cancer has no relation with infertility. What the explanation you are giving has got a theoretical background, but practically chance is negligible. Prostate cancer can not be caused by anal sex. Anal sex may increase chance of prostatitis not cancer. You may consult sexually transmitted disease specialist. </s>