Speaking generally, rather than about Prop 8, I suspect the answer to your question is "actually, yes a donation to a campaign could imply any number of alternatives".
We like to think of politics as a simple matter of supporters and opponents for the cause at hand. But in practice it seems people can be prompted by all sorts of things -- being irked by some aspect of the other campaign, a tangential issue (such as it coming from the people rather than parties), a friend or relative being involved in the campaign -- there are potentially rather a lot of reasons why someone can be prompted into donating to a campaign they might not even vote for.
Fair enough on the general statement, but in this specific case, if any of these options were true, it makes his decision to not dismiss the donation and throw another $1000 at GLAAD or some other group be all the more questionable.
I absolutely guarantee, had he said something like:
"That was 6 years ago, I did it for $rational_sounding_reason, I don't hate gay people, I'm donating to $LGBT_charity as a token of good will"
..then this entire issue would have been discarded in minutes.
Believe me, I don't want to believe that the guy who wrote JavaScript simultaneously holds backwards beliefs like this. That alone is hard to reconcile, given how inclusive the tech community generally is. But every time I approach this issue, I can't arrive at any other sensible conclusion. The confluence of:
* The donation
* His refusal to repudiate the donation 6 years later
* His refusal to directly address the issue
..does not lend itself well to any other explanation.
We like to think of politics as a simple matter of supporters and opponents for the cause at hand. But in practice it seems people can be prompted by all sorts of things -- being irked by some aspect of the other campaign, a tangential issue (such as it coming from the people rather than parties), a friend or relative being involved in the campaign -- there are potentially rather a lot of reasons why someone can be prompted into donating to a campaign they might not even vote for.