In Hindu astrology, Mangal Dosha is an astrological combination that occurs if Mars(Mangal) is in the 1st, 2nd, 4th, 7th, 8th, or 12th house of the ascendant chart. A person born in the presence of this condition is termed a manglik. it means 50% people having Manglik Dosha, Now we have to see it from Chandra Lagna also it means 50% more will be having Manglik Dosha, then we have to see it from Venus Lagna it means 50% people having Manglik Dosha in totality 150% people on earth have manglik dosha,
Mangal is very important planet in marital relationship. Contrary to what people perceive Mars has its good and bad effects. As per Western Astrology Mars is the Male energy and Venus is the female energy. Mars when placed in 1, 4, 7, 8 and 12 houses does not give similar results. Generally people equate Manglik in each house as giving same results. That is purely because of their ignorance. There are many authors who have described Mars influence in each house. I’m sure if you search on AstrologyMag.com you will find few articles on this subject.
You are correct Mars is not at all a bad planet in every condition. Yes there are some times when it can give very bad effects, and that is true with every planet. Also as far as I know Mars gives marriage during its period.