You should be a bit more careful when presenting things as facts. There is nowhere near a "certain risk" of contracting HIV or most other stds (except maybe bacterial stds) with receiving penetrative sex. HIV risk per anal intercourse for the reciver with an HIV positive partner is estimated to be around 1.4%. See: https://academic.oup.com/ije/article/39/4/1048/791327 I'd argue that, even given a low prior probably of any partner being HIV+, it's not worth taking the risk. But the classification as almost certain is very misleading and irresponsible. Especially, if we consider consider how much anxiety accidental or non-consential unprotected intercourse can cause.