I'm not sure who was forced to celebrate anything in this case. I believe the company was just asking for its workers to show respect for each other in the workplace.
In any case, the First Amendment does not apply to a private company.
Could a private company ask you to renounce your religion as a term of continued employment?
From what I read, the guy objected to a "promote diversity" poster with photos of homosexuals IN his cubicle. At any rate, the article said the guy said he'd take his quotes down if they took the "gay promotion" poster down, but no go.
The unconstitutionality I'm referring to are laws/judicial decisions forcing acceptance or normalization of homosexuality such as the law in CA mandating "gay" friendly promotion cirriculum in public schools K-12 since 2001. Just one example of many.