We all hear about this dosha often when we talk about marriage, this is one of varoius very famous doshas in vedic astrology. To have a better idea , lets see how this dosha is created in a birth chart
when a person has mars placed in either of following houses in a birth chart at the the time of birth 1st,4th,7th,8th and 12th houses, then manglik dosha forms, according to tamil astrology, if mars is in 2nd house then also the person is manglik.
now if we count the mars placement houses, it comes out to be six houses out of 12  houses of a birth chart. that means , half of population of kids born has mars placed in either of houses at any time because  ascendent keeps changing after every 2.5 hours, this shows that 50 % of population has manglik dosha in different ways.
now other way to look into manglik is to look same houses from venus of the person, which means if mars is placed in 1st,4th,7th,8th,12th houses from venus, then there will be another 50% people have this kind of dosha,
now we have 100% population manglik, !!! some astrologers also see from jupiter and moon too,
this continues on and on... never ends !! This is funny and nonsense!!!
being a manglik is nothing but the placement of mars in different signs , nakshatra, upnakshatra,good or bad aspects on it ! through which we predict incedents of the life.
another misconception is that if manglik person marries marries a non manglik then non manglik person will die or health of spouse will always suffer badly
Tell me one thing that how can we see the age of spouse and health of spouse from their partners birth chart ??
this is completely non sense , some ones age and health is only seen from their own horoscope and not from their husband or wifes birth charts.
in another misconception , people marry their daughter to vishnu or some other sacred God, or some times with a KALASH ( clay pot), then divorce their daughter with that god and then remarry her with a regular boy !!!
firstly , if you marry your daughter with god, then she is supposed to be a goddess now , and how a goddess can marry to another man after divorce from god ?
we just do it for fulfilling our stupid old empty cultures , which are totally non sense and has become a money making business for numerous astrologers and pundits
pujas doesnt help , only right thing to do is  to go to a good astrologer and do match making of both couples, and discuss the negative aspects with astrologer, he can tell that at what point, the problem comes in married life,
those problems are 99% related to ego issues, because we dont have manglik problem , we have PEOPLE problems, once the people problem is solved , doshas will be removed automatically,
The only genuine reason behind it is that Mars is a passionate planet and where ever sits in a birth chart, it imparts passion in those fields of interests in those houses, therefore if manglik marry manglik then their energy passion level will be more identical which helps in building good relationship
on the other hand , if non manglik marry a manglik, the non manglik person will feel like little hard to adjust with the energy passion level of spouse, which all depends on your understanding
so please stop fearing from MANGLIK DOSHA, it is nothing and waste of time and money making businness for fraud astrologers