Marriage with the same surname is fine as long as they have different gotras. Its highly unrecommended to marry sb with the same gotra. its not superstitious but scientific too.When ya marry ppl with some gotras, it is likely to have lot of similar kinds of genes so when ya have kids its likely for the recessive genes to express and result in some kind of genetic defects.Let me explain: you might have a gene for a certain defect but ya might just be a carrier so ya donn have the defect but after crossing with a person of nearly same genetic make up your offspring might get the recessive gene from ya and your wife which makes the defect dominant (expressed visibly).