The first thing you have to understand is that life is not all about you, and how you feel. Your mum and dad obviously love each other, and they have an open and relaxed attitude towards sex. in a conservative country like India (I assume you did in India), finding people who do not try to hide everything from their children is quite a refreshing surprise.
Half of the problems that young people have when they live in Conservative environments is that nobody talks about sex, nobody explains things to them, and they never see anything unless they are skulking around in their bedrooms watching pornography. Try to understand that your mum and dad are behaving perfectly naturally. It is you that has the problem, not them!!!
What matter is it to you whether they have another child or not.. Surely it is up to them to do what they want to do in life, and they don't need your permission, or your blessing. What is to be embarrassed about anyway? Childbirth is a natural consequence of sex, if that is what dosing reached in the act are aiming for. Your mum and dad love each other, and you, and your siblings, visible proof of the love they share for each other. You should be happy for them, not criticising them for doing something that is perfectly natural. If you don't want to see them having sex, don't look!!! Let's be honest, you can hear people engaging in sex long before you can actually see them do it, so if you can hear your mum and dad having fun together, give them the privacy that they obviously feel they don't need, but you need. On the other hand, you might actually learn something about a loving relationship, and to be honest, if you have never had sex before your wedding night, you are going to be in for a bit of a shock!!! One thing is for sure, at least you realise that sex is something to be enjoyed, not something to be frightened of.
Now, for gods sake, you're 22 years old, and you shouldn't be behaving like a frightened little schoolgirl. Nor should you expect that because your mum and dad are not in the first flush of youth, they should stop enjoying something that everybody wants to enjoy for as long as they are physically capable, as you yourself will find out one day, assuming you meet the right man!!! It is a fact of life the people don't stop having sex once I reach the age of 30. Many of them don't stop having sex when they reach the age of 60. My mother-in-law died two years ago, and she was still having sex when she was 80. She didn't look her age, even when she died from a heart attack. She had a boyfriend who was a much younger man than her, and they enjoyed themselves and each other on a frequent basis. I remember her saying to me once that she may not make it to a hundred, but she planned to carry on enjoying life to the maximum, right up to the moment she died. I loved my mother-in-law, she always accepted me without any criticism or unkindness. I married into a white English family, and they welcomed me without reservation. That's a lot more than I can say about my own family, who disowned me for disobeying their edict and refusing to marry the person they had chosen for me.
Concentrate on being happy, both for yourself, for your siblings, and for your mum and dad, who love each other, and doubtless love you and their other children. They consummate their love on a frequent basis, and what on earth is wrong with that???