Putting up a discriminatory sign is public structural discrimination and already illegal afaik, so it does not work as an example in this context of private individual discrimination. In reality it is not possible to force a homophobe person to become tolerant, no matter how many laws you make against discrimination. The only way that really helps is education and a social development towards more tolerance. Forcing christian fundamentalists to work with gay people, despite they absolutely refuse it, is not the way but would only create even more social tension and hate.