David Martin, 67, now retired, is accused of repeatedly abusing one girl over a 15-year period.
The court heard that the girl was in her 20s when the abuse stopped.
Martin, of Sycamore Grove, Cantley, has also been accused of allegedly groping another girl, now in her 30s, after calling at her house while looking for a missing boy.
Richard Thyne, prosecuting, told the court that the woman first told police about the alleged attack after she made an allegation of domestic violence against a former partner.
Martin, who appeared in court dressed in a grey suit, blue shirt and purple tie, is alleged to have sexually assaulted the girl while he spoke to her parents in the middle of carrying out his door-to-door inquiries.
The woman told police that Martin continued to show up at her home uninvited, and while she said he did not assault her again, he did make her feel uncomfortable.
All of the offences are alleged to have happened in the Doncaster area.
Martin denies 19 counts of indecent and sexual assault.
The 67-year-old served as a police constable with South Yorkshire Police and was based in the Doncaster area before retiring from the force in 2003.
The trial continues.