The Birth chart shows the various combination for a successful marriage and shukra (Venus) is the planet which promotes relationships. Position of shukra is very crucial to have any successful relation ship.
Birth Tithi is another potent tool to check the water content in your relationships. If the Birth Tithi is of a Graha which is agni tattva (if you are born on Pratipada or navami tithi ruled by surya or agni or trittiya tithi ruled by mangal then you will have severe problems in relationships).
any combination of mangal and shukra can bring violence also . If Shani is with Shukra then marriage will be full of sorrows.