See Devarim 23:18. Mizrachi - Kedeshah really means designated, sometimes for the good, sometimes for the bad - in this case she is designated for Zenus. Tosfos (Yevamos 79b) - "Lo Sihyeh Kedeshah" refers to one who cannot have Kidushin, i.e. a Shifchah.
D.A.F. thanks the following for their significant contributions towards the development of this section:
Mechon Mamre, for generously providing the English & Hebrew Biblical texts
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