Jack was sexually active, there is no doubt. I am not sure I would call him a whore. I am sure that Ennis was not Jack's first sexual contact, and after he and Ennis split up in 1963, Jack was busy in TX rodeoing and trying to pick up a rodeo clown and heiresses.
Jack had been to "Mexico" probably numerous times in the 20 years between 1963 and 1983, and probably had an ongoing dalliance with Randall - that fishing house on Lake Kemp came in handy. But, in the end, it was Ennis's shirt which was nestled in Jack's shirt, and in the closet scene in OMT's house, Ennis understood that in spite of Jack's wandering ways, it was Ennis that Jack loved, and vice versa.