Douglas Bruce is definitely pretty low in my book. And it wouldn't surprise me if he was somehow involved in Amendment 2. He definitely was involved in TABOR. But there is a bit more evil to go around.
That said, Amendment 2 didn't take away same sex marriage (earlier, CO had already heard that in courts and there was solid precedent banning same sex marriage). Amendment 2 was focused on preventing municipalities, particularly home rule municipalities, from enacting anti-discrimination employment, housing, public accommodation, etc, laws. This was a result of Boulder passing more laws to protect some people (the first laws passed in the 70s, and were related to housing discrimination). This was seen as sin and the destruction of Colorado by some. Essentially, Amendment 2 was intended to undo local laws and prevented future local laws that provided any anti-discrimination protection to gays. It was a statewide attempt to create a right to discriminate.
The official sponsor of Amendment 2 was "Colorado for Family Values." Their back-room sponsors are a who's who of right wing Christian groups, including Focus, AFA, etc. Right now a different car dealer runs what is left of it (he's from Denver, FWIW). Tancredo didn't have anything overtly to do with it, although I'm sure anti-tax and anti-gay fit together well in elections so I imagine there was some spillover. High turn out in support of either issue would help the other issue.
I remember listening to Focus on the Family broadcasts that explained they didn't want to discriminate, they just didn't want special rights for homosexuals. You couldn't listen to their programs without hearing that message. They didn't explicitly say "vote YES on Amendment 2", but it was clear that was the intent. So they were most definitely a huge factor. They also helped with the groundwork of getting local churches to mobilize their groups, by explaining to these churches how awful gays supposedly were, and how they would outlaw churches, convert kids to gay, and whatever other evil.
Same sex marriage was actually banned in 1975, due to a Boulder clerk issuing marriage licenses to 6 couples (the constitution at the time didn't prohibit it, nor did any state law). One of those couples is still married (their marriage has not been challenged - but all the ones challenged were ruled void). Someone tried to marry his horse in protest (it was denied - the horse was 8 years old, so was not yet old enough to consent). Colorado was the first state to marry gays (San Francisco has nothing on old Boulder - new Boulder is an entirely different matter). Sadly they don't allow gays to do so anymore, due to law changes.
In 96 and 97, a ban by the legislature was vetoed by the governor. In 2000, it passed and was signed by the governor.
In 2006 an initiative (or was it an voters?) was put to voters to allow civil unions. It failed. That same year, Amendment 43 banning same-sex marriage passed.
There was a legislative effort in 2011 and 2012 to pass civil unions in the legislature. It failed both times (it will almost certainly succeed in 2013, due to the Democrats taking control of the Colorado House and a very unpopular move by last term's Republican Speaker of the House).
