buggie:

married couples are somehow different than couples who aren't married?

Assuming we are talking about opposite sex couples, yes, there is a difference. Married couples have made a public statement of their commitment to each other.

buggie:

sitting down to dinner requires that the person you are romantically involved with be sitting next to you, but other types of events are fine to go alone?

Yup. That's the way the world turns.

buggie:

it's a "snub" not to invite people who aren't part of a group, that you don't even know well, to an event they really have nothing to do with when you don't have space for them?

It's considered a snub to not invite someone that social convention says should be invited.

buggie:

The LW doesn't have a problem with anyone's partners, she just can't invite them this one time. and there's no reason they have to be there.

True, there is no reason they *have* to be there. But I rather suspect that if she invites her friends and tells them that they can not bring their SOs, she is going to end up with a very small party. Because while they don't *have* to be there, chances are her friends will want them to be there.