PC John Torney, 40, told Belfast Crown Court he had "definitely not" killed his wife, Linda, nor his children, Emma, 10, and John, 13, at their home in Cookstown, Co Tyrone, in September 1994.
Torney, who said his home life was often strained and his sex life had "become rather cool", refuted prosecution claims that he was having an affair with Ailsa Millar.
Any allegation that he planned to set up home with her in Ballymena was "ridiculous".
Torney described Ms Millar as a confidante who was very easy to talk to and kept telling him to make up with his wife.
He met her about two months before the shootings. "There was quite a bit of head banging going on, telling me to get back with the wife."Reuse content