Path Dependency
Some people look at it, and instead of saying "this suck" and seeing what they can do to be more fair, they react by saying "Well you should be happy you get to be with me at all!" It's quite natural to want to preserve your privileges, especially when they are things everyone should be entitled to, but it's not fine to treat other people terribly because you're too afraid of losing them, and it's important for people to notice they're doing that, and that it's NOT "normal".
Mulling over the ideas in this thread, it occurs to me to introduce a further twist into thinking about privilege and, especially,
entitlement.
Maybe the more basic problem has less to do with giving priority to one relationship over others, but in assuming that one is entitled a particular relationship or kind of relationship. This is what I read into Tonberry's spot-on characterization of a deeply offensive attitude: "you should be happy you get to be with me at all!" This, to me, says: "I am entitled to your unquestioning allegiance or compliance in exchange for the tremendous pleasure of being with me."
Maybe this is why the dismay of frustrated unicorn-hunters is so hard to take. Some of them, at least, seem to feel cheated that the world is not giving them what they want . . . because they think they are entitled to get what they want. The world owes it to them to provide a single and suitably attractive and compliant bisexual female, on precisely the terms they offer - and she'll be lucky to have them.
Turning this back on myself, I like to think I'm open to the possibility of having more than one relationship at a time, but I don't think I'm entitled to having more than one relationship, nor am I entitled to any particular relationship with any particular person I am drawn to.
In fact, I think the odds are against me, because of choices I've already made and responsibilities I've already taken on. As already noted, I have been married for 20 years. I am committed to a career path from which it would be difficult and very risky to deviate: I have an advanced degree in an academic field, and tenure. I'm also in a position in which my "morals" may someday be subject to scrutiny, and I work in the Deep South. I have two daughters to raise, which tends to make me risk-averse.
I am not complaining. I simply note the path that led me here, and the constraints those choices now place on me. I have no sense of grievance about this, because these have been my choices in a context that has provided me - tall, straight, white, middle-class male whose native dialect is broadcast-standard American English and who is, to all appearances, in a safely conventional family situation - with any number of advantages.
I am not complaining, I have no sense of grievance, because I do not think the world owes me a girlfriend.