The Rev Ken Hutcherson, the pastor of Antioch Bible Church in the suburb of Redmond in Seattle, says Microsoft, Hewlett-Packard and other businesses that are urging passage of the measure, have underestimated the power of religious consumers.
Hutcherson said he intended to issue the boycott formally on Thursday when he appears on a conservative radio show.
On Monday he said: "We're tired of sitting around thinking that morals can be ignored in our country. This is not a threat, this is a promise. Check out the past presidential election. We made the moral issue the number-one issue."
Several companies, including Microsoft, Boeing, Hewlett Packard, and Nike, signed a letter confirming their support for the bill which would add "sexual orientation" to a Washington state law that already bans discrimination in housing, employment and insurance, based on race, gender, age, disability, religion, and marital status.
No backing down
Microsoft is restoring its support for the proposal only a year after it was criticised by gay activists across the United States after quietly dropping its endorsement.
Hutcherson, who has previously organised anti-gay marriage rallies in Seattle and Washington DC, claims that he pressured Microsoft into dropping its support for the bill last year after threatening a boycott.
The company said it took a neutral stance to focus on other issues, but later said it would support the measure in the future.
"We're tired of sitting around thinking that morals can be ignored in our country"
Asked about the boycott on Monday, Lou Gellos, a Microsoft spokesman, said the company would not change its position but declined to comment further.
Peter Conte, a spokesman for Boeing, said the company had no plans to withdraw its support for the legislation.
"The position that we have taken is one that we do feel strongly about," he said. "It is entirely consistent with our own internal practices and policies."
The bill has been introduced - and rejected - annually for nearly 30 years in the state legislature.