Kitty was finding her relationship with Piotr to be getting strained, and was becoming attracted to Doug to the extent that Piotr was getting jealous of Doug. He could see that Doug had a lot more in common with Kitty than he did (Kitty and Doug were both originally standard American middle class kids with a penchant for videogames and computers) and when he came back from Secret Wars and told her he had been unfaithful an alien, she broke up with Piotr.
She never really took her relationship with Doug any further, and in the famous NM#45 issue, when she dances with Larry Bodine, Warlock asks Doug if he minds seeing Kitty dancing with other guys when they'd bee so close, and he replies, apparently without rancour, that they were, and remained, good friends and that was as far as it went.