According to Haeberle, anal intercourse is commonly so satisfying to both parties in heterosexual relations that it is often recommended by therapists consulted for marital difficulties. (p. 362)
"Many men enjoy being masturbated by their partner while they have his penis in their rectum. He, in turn, may be glad to oblige because his knows that the anal sphincter will contract during orgasm and thus provide additional stimulation of his penis (p. 245)
Brongersma 1990, p. 413, 425
"Reactions of the muscles which normally keep the anus closed may be erotically stimulating." (p. 585) "Contractions of the anal sphincter (...) may bring contractions of the muscles that extend into the genitals and produce erection in the male..." (p. 586) or increase feelings of lust. "Many males are quickly brought to erection when a pressure is applied on the perinal surface at a point which is about midway between the anus and the scrotum. (...) Deep penetration of the rectum may stimulate the same perineal nerves, and prove to be similarly erotic." (p.585)
The rectum itself is elastic and extensible, but it is only sparsely supplied with nerves. (p. 581) [...] Stimulation of the interior of the rectum, however, "may also exert pressure upon the perineal nerve masses and it is known that this may bring specifically sexual responses with orgasm. This is frequently realized by persons who have been recipients in anal intercourse. (p. 698)
Brongersma 1990, p. 416-417
According to a French investigation, 34% of gay men enjoyed both active and passive anal intercourse; 21% wanted to be inserters exclusively, and 15% exclusively insertees.
Brongersma 1990, p. 416-417