Every planet occupying a house give its own impact on that house and so is ketu in 7th house.
Unlike rahu that believes in materialistic pleasure, ketu believes in sacrifices and have a nature to attend moksha and so it do not believe in attachment with any kind and so the relationship will be of a spiritual kind.
Not exactly ketu in the seventh house will affects married life, there are many other factors also in astrology which have to be seen, gives their impact in destroying the marriage.
In reading a horoscope and predicting about an event only one planet is not seen. The total impact of the nine planets are studied and then taken into consideration before arriving at the prediction. In considering the planets, their positions in rashi their degree, aspects, significators etc are studied.
In same way Ketu in 7th house has to be studied in term of its placement of rashi, degree and the nakshtra in which it is positioned in. The planets aspecting ketu is studied if its from a malefic or from a benefic planet. As ketu doesn't own any house in a horoscope, it acts as an agent of that house. So its depositor is studied and inference is generated about ketu.
According to KP vedic astrology a planet gives results of its nakshatra lord, the planet itself becomes the source of the result and the sublord gives the quality of the results.
So, in-spite of ketu in the 7th house, it will give result of the nakshatra lord in which it is positioned in first and the sublord will give the quality of its results.
So, to sum up in nutshell, by only ketu’s placement in 7th house, a marriage in not destroyed.
This same formula is applicable to every planets positioning in each house, so giving correct answer to such questions is difficult and only general answers can be given, as every horoscope is unique and so the real predictions will differ from person to person.
A general prediction is a rough prediction and need not 100% correct. It may be applicable to someone and not to others. So, the general prediction should not be taken very seriously and it is always advisable to seek guidance from a good astrologer who could guide you astrologically in correct way.
