Obviously Manglik is one who has Mars in 7th, 8th, 4th, 2nd and 12th houses. Normally astrologers take it as being present anywhere in the above houses irrespective of the longitudes of Mars within those houses. If it exactly tallies with the Ascendant longitude, it will be having 100% residential strength. The further it is from the Asc longitude, the effect also decreases correspondingly. Many a times Mars present in the 7th house or Rasi will actually be in the 6th or 8th house if they are not within the cusps of those houses. And most astrologers are unaware of it.
And why is Mars considered to be dangerous in a marriage?. Mars signifies sexual potency and if both have equal sexual potency you can consider it as very good. But if one has a weak sexual potency and the other highly potent, naturally the marriage will be an unhappy one. Vichitraveerya of Maha Bharata was one who was highly Manglik and due to his over indulgence in sex in his 3 wives, died when he was young and that is where Mahabharata starts when Saint Vyasa had to father Dritharashtra, Pandu and Vidura.
It is mainly for sustenance of this universe this sexual urge is there in all living beings and they get it free of effort even if it is a worm or an elephant. We check horoscopes of potential couples only to ensure that this purpose is well served. And being a Manglik is therefore not a sin, it is a measure of sexual potency and there is no need for being afraid of one who is highly Manglik. Please read my articles on navagrahastro.blogspot.in where I have exposed many myths and misconceptions in Astrology.