"According to the United States Government Accountability Office (GAO), there are 1,138 statutory provisions in which marital status is a factor in determining benefits, rights, and privileges."[1] He donated money to a cause to prevent those 1,138 benefits, rights and privileges being available to gay couples. He hasn't changed his mind, explained himself etc. He has issued a statement saying Mozilla employees will get benefits, rights and privileges within the company equally. But those same employees (not to mention Mozilla's customer base) also have lives outside of the company. It is very easy to see how that can be considered "toxic political action" as well as calling into doubt his suitably going forward.

[1] https://en.wikipedia.org/wiki/Rights_and_responsibilities_of...