This means to decide first on the conclusion, and then focus on evidence that supports it, and so discounting any contrary evidence. It’s closely linked to the cognitive distortion of “mental filter”, i.e. seeing what you want, discounting the positive etc.
The most common way this would manifest in the various guises of Forever Alone communities is having a fixed idea of what the desired gender is (I won’t say opposite as not everyone wants the opposite). Many people have a very fixed idea in their mind about what a partner would be like, what they are looking for, and if people have been isolated for a very long time, then this picture can be very distorted. Also, there can sometimes be very little actual contact with members of the desired gender, and so ideas about them are essentially inductive arguments based on media, talking to other people in online communities (the echo chamber) and then people engage in the sharp shooter fallacy by selecting various scenarios, media stories, memes that back up the ideas about the desired gender, usually based around how unobtainable they are.