Learn English – What’s a neutral/negative word for ‘impress’

I think the most neutral word is indeed gay, which can be used for both sexes (even though gay woman is not widely used, in favour of lesbian). From Wikipedia:

The most common terms are Gay (both men and women) and Lesbian (women only). [...]

Some organizations (e.g. Safe Schools Coalition) discourage the usage of homosexual in everyday usage, as it might sound too clinical:

Homosexual: Avoid this term; it is clinical, distancing and archaic. Sometimes appropriate in referring to behavior (although same-sex is the preferred adj.). When referring to people, as opposed to behavior, homosexual is considered derogatory and the terms gay and lesbian are preferred, at least in the Northwest.

As Jolenealaska says, we have no such word. Non-mothers is perfectly acceptable and understandable; but as you discern, it’s sort of clunky.

But since you have already identified the category with which this category is contrasted, you need not give it an explicit name. Just write:

[Herb] increases the quantity of breast milk in lactating mothers, but other women can also take the herb ...

I’ve added lactating, since you presumably want to include mothers of older children in your second category.