Anal sex. Some people enjoy it and some people hate it. Because the anus is not wet most couples use lube to get the stiff penis in the anus easier. But anal sex is not always intercourse. Men love to be fingered in the ass to pleasure their prostates.
But also anilingus or rimjobs can be named here. It means oral sex on the anus. When a woman penetrates the man with a dildo we say that she is pegging him.