I was first started thinking down this route by a throw-away comment from kissycat1000 yesterday which was along the lines of the fact that I was illogical in some ways (and stubborn, but that's not really the point I'm trying to make).
Then, reading a couple of posts including this post by stephmog, I came to realise that no, I'm not logical, and here's why.
In a monogamous lifestyle, any relationship you start is doomed to failure, except for the last one. As you have no way of finding out if any particular relationship is the last you will ever have until you have it (uncertainty principle anyone?) the best way to avoid failure is simply not to start any relationships (if you're monogamous).
But I still persist in pursuing relationships - which is clearly not logical. This means that my human needs are more important than logic - so I would suggest that no one is logical who follows logic, and anyone that does is illogical (and also sad, deluded and giving in to their fears). It's all just common sense really ;)
 Okay, now this breaks down because I'm not monogamous (in theory), but I've yet to decide if other relationships I have will be secondary or primary, but I've a strong feeling they will be secondary, in which case this argument, which is about a life partner, still applies.