There are important rules about what to do if three people are socializing together and two of them are "a couple." They just mess me up every time for some reason, and I feel there is no way to negotiate them gracefully AT ALL.
First, if you are in a restaurant or some place where you have to sit down, you have to make sure the couple sits together and the third person sits across from them. Those are just the rules, and if you mess this up, it will ALWAYS be awkward, and people will NEVER accept it--something I've learned from years of personal experience on both sides of this tense situation.
Restaurant seating arrangements used to be my worst enemy when it came to "the couples crisis," but lately I've started to feel that places like the subway or theater, where you have to sit three in a row, are even trickier. You'd think three-in-row situations would be easy because they have an obvious "egalitarian" solution: the member of the couple who knows the third person best sits in the middle, duh. BUT, this only works well when it's very clear which member of the couple knows the third person best. Otherwise, it's a mess: as a member of the couple you have to make uncomfortable assumptions about the third person and which of you he or she would rather sit next to; as the third person, you are forced to pick sides.
Do you sit next to the member of the sex you're attracted to? Do you pointedly avoid sitting next to the member of the sex you're attracted to? What if you just want to sit next to the person you've known longer, but it's interpreted as you trying to "steal" the person? Or what if you actually just like one of the people better, not even in a sexual way--surely it is impolite to express this preference openly? Or maybe not? What about if you are gay and the couple is straight, do you just automatically sit next to the same-sex person anyway? Does this perpetuate "heterosexism" and just plain sexism in the world somehow, assuming that men cannot be friends with women, etc?
In short, three-in-a-row situations require some in-depth strategy (for example, I always try to make sure I am lined up in an inoffensive way right as I enter a theater, as if I'm just going to plop down "wherever," and I've learned to stand up in subways). Yet, I thought I had mastered restaurants up until today.
Like an autistic person who has memorized different facial expressions (so I have read), whenever I've been in a couple+1 restaurant situation lately, in any capacity, I have always been the first person to sit down, amazed by my social facility, proud of my mastery--earned through bitter experience--of the simple rule of "the couple sits together and the third person sits across." Yet, today, I was in a restaurant with a couple and I liked both people (while knowing the male individual somewhat better). And then, the female individual completely messed me up by proceeding to just sit down across from her romantic partner instead of next to him!!! I didn't know what to do! I only had a few seconds to make a decision.
I wasn't thinking very clearly, and I made the wrong decision. If the rule of "the couple must sit together" is broken, then the rule is "two people of the same sex must always sit together across from the person of a different sex," no matter what. That is a true rule.