Quote from: Artscroll's Chumash (38:8)
When the call went out for contributions, the women came with their copper mirrors & piled them up... Moshe was reluctant to accept such gifts for the Mishkan because they had been used to incite lust. Hashem told him that he was wrong, however, because these very same mirrors had been instrumental in the survival of the nation. In Mitzraim, the man had come home in the evening exhausted after a long day of backbreaking labor in the fields and the women had used their mirrors to entice them to continue normal family life.Thanks to this legions of Jewish children were born.
Hashem said, not only should they be accepted, they should be used in their entirety to make the Kiyor.... The Kiyor was unique in that it would be used in the future to bring peace between husband and wife by proving the innocence of women accused of adultery.
Lesson: Even lust can be used to serve Hashem. All in its proper place!