We recently bought a female cat for our male cat. Female took 3 days to stop hissing whenever she sees the male cat. Male cat was trying to approach her since day one but today he got the chance to mate. I haven't seen him but I have been informed that after mating he is trying again and again to approach female for mating. He has already done it few times. They are my first pets so I dont know much about such behavior. What is the cause? Is this normal? This is his first mate.
Welcome to having cats! Great that you are willing give your cat the best live you can think of!
Now, cats in the wild are solitary when adults. That usually means they do quite well when kept alone. But we humans are very sociable creatures and we like to keep company. And we project that to cats. We have been doing that for such a long time that cats are actually getting more social. There has been written a lot of introducing cats to one another, I will not go into that here.
Cats can be great pets, but when fed well and given the opportunity to mate, will do so. And if memory serves me, can have 3 litters a year. With a litter size of 5 being normal, that is +15 cats a year from 2 parents. Now, kittens are great fun, but that much will have a problem finding good homes.
That, and some other reasons mentioned below, will make it clear that neutering your cats is a very good idea. Let me quote wikipedia on cats, spayed or castrated;
They can be surgically sterilized (spayed or castrated) as early as 7 weeks to limit unwanted reproduction. This surgery also prevents undesirable sex-related behavior, such as aggression, territory marking (spraying urine) in males and yowling (calling) in females.
In the US, about 80% of household cats are neutered.
To answer your question, yes, this is normal behaviour, and your female cat should be able to hold her own against your male cat. This mating will (in all likelihood) result in kittens. But please, get them neutered, preferably both!
Suggested reading for further cat care: (there are more, but these look good to me)