Too many times people say things like this when what they really mean is "respect my beliefs", and especially often when it's about religious beliefs. I know a lot of people feel their religion as a part of who they are, not only something they do. A lot of them even have good memories related in some ways with their religion.
Most times those beliefs are what they were told since little children who believed whatever their parents told them, but there is time enough for that part.
The point of this post is specifically that beliefs are not something to be respected. None. But why?
Let's start with some easy ones. Think about the times you've said or been said that "you have to respect other people's beliefs" line and now think about beliefs you or they would never agree with. Like black people being inferior to white people, or women being inferior to men, or women being property, or Nazism.
If you are in a group believing the above, think about the belief that all people should be equal before the law, regardless of race or gender. Now it's easier to see that at least some beliefs don't deserve respect, right?
Now think about the occasions of that respect-beliefs sentence and realize most happen when someone openly disagrees or challenge them, often emphasizing whatever bad things it has brought in the past. That's related but a bit out of today's point. This paragraph's idea is to realize that any challenge to some beliefs is considered disrespectful by believers, so what that sentence usually mean is "you shouldn't challenge other people's beliefs"
I guess that taboo is precisely because religion or similarly held beliefs are considered a part of the believer's identity, at least by the believer, and we are supposed to respect who other people is.
First of all, no belief should be considered part of anybody's identity especially by that somebody. A belief is, by definition, something we think is true or probable, but that we can't prove, and usually something we can find arguments against. That means a choice even when usually it's not an intellectual choice but an emotional one, and as a choice is susceptible to change. That is why we shouldn't held somebody's belief as their identity.
Once we've taken that out of the way, let's focus on the evidence part. Nothing that can't be proved with enough logic should remain unchallenged. Nothing at all. If among the reasons to belief something is that it's nice to believe it, that is reason enough to doubt that belief and challenge it as soon and as hard as possible. At the very least, we should acknowledge, to ourselves if not to anyone else, that it's an opinion and that we are not trying to make other people see the truth but to convince them of our own opinion.
That is not going to prevent violence: the opinion that those things should be mine, yours or theirs have historically being argued violently. Opinions like some people are not other people's property has been violently defended, mostly by the ones being considered property. And of course the opposite opinion has been so by the aspirants to owners.
So, opinions are not to be part of what a person is, opinions are not truth, and opinions don't prevent violence in their sake. Now, what else?
Opinions should be challenged. Opinions must be challenged. First of all, by their owners and defenders, then by everyone else. No opinion deserves special treatment. No belief deserves not to be challenged, debated, argued. And, above all, opinions about how a particular god wants you to act. That's religions, the beliefs most usually getting the "respect my beliefs".
No, you don't have to respect anybody's beliefs. You should challenge every belief.