I don't consider anything cheating except oral sex or penetration with another person outside a committed relationship when it has been mutually agreed that the terms of the relationship do not include sex with other people.
Magazines, videos, phone sex, cybersex, masturbation, strip clubs...have at it, within reason (to be defined by the couple). If my boyfriend is having cybersex repeatedly with the same person, or if he is spending too much time with all these other things, then we have to talk because it points to problems in the relationship. The occasional thrill, though, can actually keep the home fires warm, in my experience.