Years ago, one of the first couples we met to play with. The guy was hot for L (hey, most guys are) and L was ready to play. His wife did not seem to be into it.
We met them at a bar, at their request. L and I had water, the husband had a beer. The wife had 4 doubles of some killer drink (zombie? stinger? something with about 6 kinds of liquor in it). Not sure if the wife wasn't interested in me or just wasn't interested in swapping. After chugging her fourth, she declared we should head for the motel.
On arrival, the wife disappeared into the bathroom. After 10 minutes, the husband went in after her. L and I shrugged. We couldn't leave gracefully, as it was our room. 10 minutes later they came out with the wife in a pricey outfit that would have been sexy on a sober woman who wanted to play.
L and the husband got naked and started playing. Even though the wife didn't seem interested -- heck, she was barely conscious! -- she seemed insulted by my reluctance, so I felt like I had to do something. You could tell her body enjoyed the sex, but her mind wasn't even in the same county. Not a stellar experience for me. It wasn't that she was physically unattractive, it was that she was uninterested. On the other bed, L and the husband had a few good, albeit fast, fucks. Neither the wife nor I initiated anything further with each other.
Since then, L and I have been harder-nosed about not playing with couples unless the wife convinces us she's interested.