* Eich was ousted from a full-time job. Thiel is not an employee of YC.
We don't normally consider CEOs to be the sort of person where we have to worry about their ability to find another job.
* Eich's beliefs were well within the mainstream; Barack Obama campaigned in opposition to marriage equality.
That's a difference of degree, and a smaller one than we would both prefer; Trump did manage to get nominated, after all. I don't think you can say (alas) that Trumpism is completely out of the mainstream in 2016.
* Eich's support for Prop 8 was quiet.
True, but does it really count for much?
* Eich's support for Prop 8 was relatively insignificant.
Another difference of degree. For all we know, Eich may have donated a larger fraction of his bank account than Thiel did.
* For that matter, marriage equality is a substantially different issue than Trumpism.
Marriage equality is one issue and Trump has raised many, that's true. But I think the attitudes in which Prop. 8 support was rooted, and those in which Trumpism is rooted, are essentially identical. For instance, a rearguard action against marriage equality is still being fought in Alabama, and I expect my relatives there are big supporters of both Roy Moore [0] and Trump. In fact it's interesting, now that I think of it, that Trump has not attacked gays; it would certainly play well with much of his audience. Maybe it says something about how far we've come, or maybe he's just too busy attacking Mexicans, Muslims, and women to bother with gays.
A better argument might be that Prop. 8 was just a return to the status quo ante, while Trumpism is an existential threat to the nation. Why a smart man like Peter Thiel can't see that is beyond me too. But I can see the argument that Thiel is not evil, just deluded.
(P.S. You're a brave man, using asterisks on HN :-)
[0] http://www.al.com/news/birmingham/index.ssf/2016/10/lawyer_t...