Under New York law, a separate property asset does not become marital property just because the parties were a couple at the time the property was purchased. Revisiting the case of a house which was purchased by one party before the marriage, which is separate property, an unfortunate realization for many people is that living together in a house one person purchased and owned before the marriage does not make it a marital asset once the parties marry. So, cohabitation in the house before marriage does not transmute an asset from separate to marital.
One particularly interesting case which touches upon this situation is Zelnik v. Zelnik, 169 A.D.2d 317 (1st Dept. 1991). In that case, the husband acquired a home before the marriage that was deemed to be separate property. This decision clearly articulates that the house was separate property even though the couple was cohabitating at the time the house was purchased, although they were not married.