I don’t remember what it was called, but the general plot:
Wife’s mother/protagonist mother agree that they’ve had long enough – it’s time for the wife to get pregnant. They arrange for a bull to breed her while his mother gives him a handjob. Husband and wife are reluctant, lots of talk about a “real man” breeding her and how he’s had long enough, so something must be wrong.
Source: reddit.com/r/Erotica/comments/mx8lc2/request_porn_story_mother_in_law_and_mother