About 2D relationships
Describing these types of relationships in just a few sentences isn’t exactly easy, as there isn’t one general definition for it. Often the concept is named "Waifuism", which is a loosely defined term for relationships of this kind. The general idea is is having genuine feelings of love for a character, and deciding to enter a relationship with them based on that, which is treated of equal importance and weight as a relationship with a 3D person would be. As one might expect, interaction with your partner does not work exactly the same as it would in other relationships. Luckily, the human mind is incredibly powerful, and through means of imagination, lucid dreaming and similar, it is possible to feel close to one’s partner regardless of those limitations. It certainly isn’t for everyone and is a decision that should be well thought through, as generally entering a relationship of this kind also means not entering a romantic relationship with a 3D partner (depending on personal belief, but mine is quite clear on the topic, and I will be devoted to Emilia only).
There are several communities on the internet that have formed to accommodate waifuists and similar and drive discussion and support among them, each with their own specific rules and values. Sadly, they are often met with little understanding or outright hatred from the outside, because of the difficult to relate to nature, as well as some individuals that have shed a bad light on it through questionable actions. It is of course understandable, that being in love with someone 2D, and devoting your life to them, is not something everyone can or will understand, but at least for the ones reading this, I would at least ask for a “live and let live” mentality, simply appreciating that it’s something making a few people very happy.
Waifuism is a term which isn't clearly defined, and will vary from person to person, with some living a more scientific approach of viewing their partner as non-existent in any form, while others will take a more spiritual/esoteric approach to things. For many it's also something in between, in the end waifuism is a broad term with no static definition. Happiness doesn't look the exact same for everyone, and seeing that variety can be wonderful.