The genus Lilium contains many species that can cause gastric distress if ingested. Apparently, cats are super-sensitive to some of these toxins.
Many plants in nature have evolved defense mechanisms to ward off or resist predation by whatever wishes to consume them. My reference (“Toxic Plants”) states: “It appears that many species of Lilium are a risk to cats but not dogs. This may simply be due to a relatively higher dosage required for other animal species.” The particular toxins in lilies to which cats succumb are not well studied, but oxalate crystals, tannins, saponins, and furanones appear to be involved.
Most grasses are harmless, but many other plants are not. So do not let your pets eat the vegetation around the house. Some plants of particular concern besides lilies are: oleander, foxglove, holly berries, daffodil, tomato vine, bird-of-paradise flower, flowering sweet peas, lantana plants and berries, Jerusalem cherry, iris, hydrangea, ornamental asparagus, caster bean, rhubarb leaves, and potato vines. There are many more, also some persons and animals have allergies to particular plants. Therefore, if you are not sure, ask for information from your veterinarian, doctor, and/or a plant expert.