I think your parents are the only two people that can best answer your question. People on this forum are very helpful, they will answer your question, no doubt about it but, their answers will not have any impact on you. Why? The answer is very simple. We listen to those that we love, respect and know especially when they happen to be our parents. Strangers don't have the same impact on us as our parents or loved ones do. You claim you are aware of the history between religions then why are you so 'baffled' at your parents decision not to accept this muslim man? What does this mean? Does it mean you haven't learned any lessons from our history? Which 'few rotten apples' are you referring to? What 'bunch' are they spoiling? I am not entirely sure what is it that you are referring to. If you knew the sikh history as you claim you do, you would have soon learned that our glorious history is much larger than few rotten apples and a bunch they are meant to be spoiling! Our history is very unique and powerful and to express it in terms of few rotten apples and a bunch is absolutely sacrilegious! There is no history in this world that can equal the sikh history. Believe you me our sikh history has got nothing to do with few 'rotten apples,' or a 'bunch' and to see it being expressed in this way is hurting me so much that I am literally in tears now.
Please read the SGGS, the Dasam Patshah ji's Bani, about Mahaan Baba Deep Singh Ji. Find out about Dasam Pita's sacrifices for us so we can call ourselves sikhs today. We are heavily indebted to our Deshmesh Pita for all the sacrifices that have been made for us. Do you know why our Deshmesh Pita ji is known as 'Sarbans Daani?' There is no way we can ever repay Him. If you are lucky you will soon learn that our history is more than just 'few rotten apples' and a 'bunch' they are suppose to be spoiling. There is simply no comparison.
I am not sure if you are a sikh. In my view, it is arrogant to call oneself a sikh when you don't even know your history which is replete with painful martydoms, imprisonment and sacrifices throughout, starting with the first Guru to the tenth!! Besides knowing our history it is also imperative to learn lessons from it!