First, before I start, I must disagree with some of the attempts to placate the sensors when MBTI "experts" ect state that intution does not equal intelligence. Based on my experience with sensors and intuitives, I have found that intutives are generally quicker on the uptake than sensors. There are some exceptions-some of the logical sensing types can be fairly bright, but they often are below the higher superior ranges where many NF and especially NT's (due to the powerful logic/intuitive combination) make up the majority.
SF's often seem to the least intellectually capable. Of course, there are exceptions, but I doubt, for example, that any belong to MENSA, whereas many INTJ's probably do. Thus, a typical relationship between an INTJ and an ESFJ could be seperated by 1-6 standard devations in IQ points. This has to result in significant problems in these dual relationships. While a smarter ESFJ would be intellectual compatable with a slower witted INTJ, it seems the vast majority of these types would pair up and soon become disappointed in the large discrepency in brain power.
I this the case in many NT-SF dual relations, as NT's are often the smartest (although NF's are often equally bright), and SF's are often the dimmest (of course, not always though), these problems in intellectual difference are most drastic, because in the other dual relations an intutive feeler is teamed up with a sensory thinker, and thus each partner possesses on of the necessary strong function that results in higher levels of intelligence.
As a result, many NT's probably marry other NT's because of intellectual compatibilty.
I do not mean to offend anyone. Any remarks are appreciated.