What semantic notions underlie 草 (grass) with 拈花惹草 (philandering)? How does grass semantically shift to involve womanizing? I know that women have been metaphorized as flowers, but not grass? Wiktionary. 拈花惹草的解释

lit. to pick the flowers and trample the grass (idiom)​, fig. to womanize, to frequent brothels, to sow one's wild oats

My research on Flower Symbolism as Female Sexual Metaphor by Andrea Frownfelter, Eastern Michigan University.
Erotic blooms: The sex appeal of flowers - BBC Culture

Between flowers and sex there has long been an enduring link. In the classical age, women (especially virgins) were compared to flowers, whether in Virgil’s agricultural Georgics or else in Sappho – who, in one fragment, makes an analogy between a woman, perhaps married, and “a hyacinth in the mountains that the shepherds trample with their feet.”

Titian's Sacred and Profane Love: The Feminine as Flower: Metaphor and Hermeneutics:

When the feminine is referred to and portrayed through the flower metaphor, it often means to reference the vulva as the primary sexual characteristic of the female and/or more broadly, to that creative force which rises from the vivifying power of the body.

L.P. Murashova, L.V. Pravikova. EROTIZATION AS THE BASIS OF FEMALE METAPHOR IN THE ENGLISH LANGUAGE. DOI: 10.17223/24109266/4/3. p 30.

      Associations of women with flowers as metaphors of fertility are old
and are used in many languages. Flowers are used as both positive and negative
symbols of female sexuality. Instrumental and symbolic role of women
are projected on “universal” values of motherhood and femininity in form of
fetishised body parts of a woman. The association of woman with flower
evokes a vision of a defenseless beautiful creature in need of protection.
       Floristic terminology, bearing floral symbolism, often uses parallel analog
correlation between flowers or their parts and aspects of female sexual
anatomy, such as the bosom, womb, labia and vagina, and also nominates the
stages of women’s sexual and reproductive activity and sexual status and actions.
If we extrapolate this symbolism further, mankind is born from the
womb of a woman, which means that the sexual and reproductive organs of
women are the “essence” or “flower” of humanity. Puberty, female physiological
processes that happen in the body, first sexual experience, fertility, pregnancy
and motherhood are conceptualized through floristic terms: enclosed
buds, ripe blossoms. The lexeme “flowering” is sometimes used to describe a
particular period for a woman - in the phrase “menstrual flowering”.
“Rosebud” is a nomination of a beautiful blossoming young
The focus in 拈花惹草 is not 拈花 (touch flower), but 拈野花 (touch wildflower) and 惹草 (contact grass) actually mean (touch wild grass).
野花野草 (wildflower and  wild grass) is a collective noun for 'wild plants' (outside women) in contrast to 'homegrown plant' (your wife)