buggie:

The fact of the matter is that different rules apply to single people than to couple people, whether they be etiquette rules or tax laws.

Yup. Rules are different for different people in different situations.

buggie:

There's no rule that says that if you are inviting a single person to a dinner party you have to invite them to bring along a friend or relative as well, so why is there a rule about coupled people?

Because human nature being what it is, a person who is in a couple desires to spend a large part of time with their significant other. When it comes to formal events such as weddings, dinner parties and the like, bringing your significant other is an affirmation of the relationship and indicates the level of importance it has in your life.

buggie:

It's putting more importance on romantic relationships over other relationships.

Romantic relationships are generally more important than other adult relationships.

buggie:

Furthermore, it sets the two groups apart, one with privilege over another.

No, it doesn't. A single person can choose to have a significant other and then the rules apply. It's like me complaining that a woman who has a child gets special privilege because they can get tax breaks for their children and bring them to children's parties. I have chosen not to have children, and thus I don't participate in the child having part of society.

buggie:

lso, the suggestions that the LW split up the dinner party or rearrange the purpose of the event suggest that upholding the rules of coupledom trump the friendships that are already in place.

Ah, but the coupledom rules do trump the friendships that are already in place. That's not to say that person should dump her friends the instant she gets a boyfriend. But a long term relationship (especially one that leads to marriage) indicates that the SO is the most important adult person that person's life.

buggie:

First of all, I hate this term SO. It implies that no one else in the person's life is significant.

Until same sex marriage is legalized, there is no other good gender neutral term that one can use without being heteronormative. And, when someone is in a long term relationship that either leads to marriage (or is essentially marriage-like) that person is the most significant adult person in that person's life.

buggie:

You're whole argument here is based on the assumption that everyone who has a romantic partner, wants to take them along everywhere, even if the situation has nothing to do with them, AND that no one would want to bring anyone besides a romantic partner along to anything, AND that even if someone is so petty and dependent that they can't leave the house without their romantic partner and/or is offended that they are not invited to something that again, they have nothing to do with, then their wishes should upheld just because we are talking about romantic partners, and it's "social convention."

Holy run-on sentence batman. A person who is in a romantic relationship does want to take along their SO almost every where. If they didn't, they wouldn't be in a romantic relationship. When you share in someone's life like this, there is very little that has "nothing to do with them". I go to my husband's work functions. He goes to mine. I go to parties his friends throw. He comes to mind. We are sharing a life. Now, I don't bring him to the "woman in physics wine and cheese" because that wouldn't be appropriate. But, if I didn't want him around to share my life with me, I wouldn't have married him. Sure, someone may want to bring someone other than a romantic partner to some event and they can ask the hostess if cousin Fred can come, but let's face it, they're not sharing their life with cousin Fred unless they're from West Virginia. Certainly people can go to events without their SO. Most people do. But, they generally choose not to unless the event would be inappropriate (such as a girls night out) or the SO is busy or completely uninterested. People generally pair up. It's instinct and human nature. You are certainly free to insist that none of your coupled friends can bring their SOs to your events. But, I think you'd find that you wouldn't have any more coupled friends after a short while. That's just as exclusionary as couples who don't invite their single friends to things like "dinner parties" which are viewed as more couple-y events.