How is harmful to keep laptop on the legs or near of the body in terms of magnetic influence?
Magnetic, no. As Variax points out.
A potential risk would be one of heat from the battery and components within the laptop itself, which gets hotter as the laptop works harder, especially if the cooling vents are blocked (by clothing, cushions, cats, etc.).
Heat is a form of radiation, but it's just that - heat.
The electromagnetic radiation emitted by a laptop is completely harmless, both by its amount and by its spectrum. No harmful effects occur from this kind of exposition, regardless of the affected area or the time of exposure.
The heat can increase scrotal temperature, which may influence male fertility 1. However, that study is from 2005 and laptops are much less hot these days so I would not think its that much of an issue...