By that same rationale, the boycotts on Target and Best Buy are just as invalid. Target and Best Buy also have ratings of 100 on the corporate equality index. HRC's corporate equality index does not take into account the donations of the owners or executives.
For example,
Brent Andrus, one of the executives of Huntington Hotel Group (a holding company for Mariott Hotels) is one of the largest individual donors to support prop 8, at $20,000.
As DLNYC has stated, it's just the close ties between Mariott and the LDS church that make me unlikely to stay at their establishments, if there are other options available.