Or sent by the couple's relationship therapist (without their knowledge), and the bondage was part of the therapy.
Well yes, as an experienced therapist, he has developed an eye for those couples,
where he thinks, all they need to patch things up is a little kink in their life,
but they're too uptight to ever try it out on their own or follow advice to do so ...
consequently he subjects them to a little bit of 'shock therapy' which enables them to ummmm, bond in a new situation...
Actually, home invasion situations have always been a fantasy of mine.
Not an incidental burglary, but intentional.
To put the couple under pressure, because the invaders want something specific from them.
Or intimidation of political opponents by an authoritarian government.
I do have a similar fantasy (though not usually with couples included, just me),
where the invader is sent to basically, deliver a harsh warning ...
... its made to look like a random home invasion & rape,
but it's because of something I've done,
and 'they' want to send the message they're on to me,
an intimidation tactic as you said,
without going to the effort of actually assassinating me or anything
( as I am not that important,
and in whatever scenario I'm imagining,
the police is not yet completely corrupted,
and might actually ask a few questions about a body
usually I find some way to make it turn out worse than intended!
... touched upon briefly here ...
http://www.cruxforums.com/xf/threads/fear-before.7160/page-10#post-540475