This is an interesting question, which feels like several interesting questions in disguise. My instinct is to start answering your question by noticing the different questions that seem to be lurking underneath it. I read at least three questions here,
- What's the connection between sexual orientation and sexual activity? Can you call yourself X if you haven't Y'd?
- What are the defining features of sexual orientation? That is, in this case, what makes a bisexual a bisexual?
- Who gets to call themselves what? Or in your words, when is it proper to call yourself something?
Let's start with your first question first. There isn't a precise or clear relationship between sexual orientation (in this case, bisexuality) and sexual activities (which I want to define as whether or not someone has engaged in any activity they or their partner consider to be sex). Having sex, whatever that looks like, doesn't make us anything. It's a behavior we are engaging in. Sexual orientation may have many definitions (see below) but it isn't the same thing as sexual activity. So, for example, it is a common misconception that all men who identify, or are identified, as gay are into anal sex, and any man interested in anal sex either is, or might be, gay. Neither of these things can be said to be generally true. Lots of men who identify as gay aren't that into anal sex. And lots of men who don't identify as gay are. The distinction to be made here is that sexual activities are what you do, and sexual orientation describes who you want to do things with. Of course we might describe the "who" in any number of ways. Sexual orientation focuses only on gender. Which brings us to your second question. What defines sexual orientation?
Consider where this word comes from. Sexual orientation originally referred to how one positioned oneself in terms of a sexual goal (which might have been something you wanted to try, someone you wanted to be with, some way you wanted to be known as). In this way, we each had our own unique sexual orientation to others and to the world. The way we use sexual orientation today comes from a medical model or approach to sexuality. This approach is the one favored by most sex educators, researchers, medical doctors, and an ever growing army of sex pundits. The medical model relies on the understanding of sexuality as something we can measure, and know, and evaluate objectively. In this model individuals may have their own interests, but we can all be categorized and defined using labels that apply to everyone. The essential falsehood about these labels is that they are objective or scientific. They are, of course, as value laden and tied to things like race, class, and culture, as anything else. Sexual orientation is just another label.
Sexual orientation today is used to categorize people based on the gender of the people they are primarily sexually attracted to. The defining characteristic of sexual orientation is not what someone does sexually, or wants to do sexually, nor is it what they call themselves, it is solely a reflection of their identified gender and the gender of the people they are most interested in being sexual with. For someone to be considered bisexual by the powers that be, all they need is to be someone who is sexually attracted to people who identify as men and women. This brings us to the last question, which is really about whether or not we want to let the powers that be define these things for us in the first place.
Who Gets to Call Themselves What?
The way you asked this question "is it proper" for someone to call themselves something, may read at first like a question of fact. Maybe even a scientific question. But it's neither. The question you're asking, which is an excellent one, is philosophical, moral, and ethical (more how do I feel about eating meat? than do seat belts save lives?). It's a question most people don't bother to ask themselves, but I believe we'd be better off if everyone did, because it's a question we all need to decide for ourselves.
Annoying as it is, I want to answer your question with one of my own: do you believe people have the right to identify themselves in ways that fit them as individuals? In this context, does someone have the right to call themselves bisexual because that's how they identify themselves or that's who they feel themselves to be? Or do you think they need "proof" of their orientation? Do they need someone else, a professional, who applies a label based on definitions developed for everyone?
If you prefer the latter option, you have to ask yourself what proof would satisfy you? And should you, or anyone else, be in a position to demand such proof in the first place? What does it mean to give any individual, organization, or government the power to tell us who we are? I know these questions may not seem nearly as fun as trying to figure out which celebrity or which one of your friends is "really gay" but if you're interested in treating others like people, and not stereotypes, these are the questions you need to ask yourself.
The Bottom Line:
Using the current medical and public health definitions of sexual orientation, what defines someone as bisexual is determined by their gender and the gender of the people they are interested in having sex with (whether or not they have had, or plan to have, sex). But thought of another way, sexual orientation is just another label that people use, and if it is one that feels good, and helps someone think about their experience and/or explain it to others, than there's no reason for them not to use it. But if choosing a label feels like an imposition, just doesn't feel right, individuals have the right to resist doing so and describing themselves and their sexual interests in ways that are unique and personal to them.