Living in a diverse society requires tolerance and respect not just of a variety of identities and conditions (race, gender, national origin, sexual orientation) but also of a variety of beliefs and attitudes (religion, philosophy, politics).
For a lot of people, it's the second kind of tolerance that's really hard. Because they think: "But those viewpoints are WRONG! They're damaging! They deny my right to exist! If I give them even basic respect that means I'm signifying my approval of them!"