Homosexuals are susceptible to AIDS, which can be transmitted through same-sex sexual activity. If heterosexual sex is normal, it will only touch the vaginal mucosa, which is a compound squamous epithelium, the same type of epithelium as the skin, and is relatively pliable, so it is unlikely that physical stimulation will cause a break. If same-sex sexual intercourse touches the rectal mucosa, the rectal mucosa is a columnar epithelium, which may be broken due to physical stimulation, and the wound will come into contact with the blood circulation, and the body fluid containing HIV will enter the blood circulation directly through the wound, which is more likely to lead to the transmission of AIDS than direct contact with the mucosa. If heterosexual sex is not the normal way, but contact with the rectal mucosa, it may also lead to the transmission of AIDS.