Double meaning: The author's lover swears an 'oath' to them of love, a promise, but since that promise is false, according to the final line of the stanza, it also resembles a curse.

The author implies falseness in their lover, since she appears as a lily, but masks herself in another flower's coloring.

In calling their (female) lover a 'lecher', the author not only implies a gross unchasteness, they imply a masculinity, since lecher was most frequently applied to men.

Rosati, G., Domenech, L., Chazarreta, A., & Maguire, T. (2020). Capturing and analyzing social representations. A first application of Natural Language Processing techniques to reader’s comments in COVID-19 news. Argentina, 2020 [Preprint]. SocArXiv. https://doi.org/10.31235/osf.io/3pcdu

Kennedy, B., Atari, M., Davani, A. M., Hoover, J., Omrani, A., Graham, J., & Dehghani, M. (2020, May 7). Moral Concerns are Differentially Observable in Language. https://doi.org/10.31234/osf.io/uqmty

These ghosts are representative of past lovers who are haunting her, which is especially disturbing to the speaker because she cannot remember them. Perhaps she cannot remember the past lovers because she was promiscuous rather than trying to find real love. I think Millay chose the word ghost because usually things that haunt you are things that you feel guilty about, and I think Millay feels guilty for her past behavior of being promiscuous.