Ford Motor Co. said Monday that its Jaguar and Land Rover brands will stop advertising in magazines that cater to gay and lesbian people, but the automaker denied that it struck a secret deal with a conservative Christian group to pull the ads to avert a boycott. Jeff Stoltman, a Wayne State University marketing professor, said the move by Ford could backfire. "They'd be well advised to create the impression this decision to shift advertising dollars in not linked with the AFA [American Family Assoc. (conservative group)]," said Stoltman.