adorable, would you give up your bashert if he was an only child? Is it up to you to decide that it is NOT convenient to do so if that is Hashem’s plan for you? Everyone comes with pro’s and con’s. Would you refuse to marry a boy because he had only sisters and no brothers? That could be a huge challenge even more so than being an only child. Can you imagine how spoiled such a prince could be?
I have had this discussion many times, and it is foolish. I have advised young people to invite the in-laws to their parents home for yomim tovim so they shouldn’t be alone and so they shouldn’t be obligated to have every yom tov with their in-laws. It is a win, win situation. Making such in-laws a part of your own family makes them happy and makes them feel as if they too have a large family and are not limited to just one child and their children. They are very grateful to be included as an important member of the family. They will appreciate you more and love you more for that. You do not have to keep them separate and apart from the rest of you. And why do you say your children will not have aunts and uncles nor first cousins? Don’t you have brothers and sisters?
In addition, when one is an only child they are usually very, very close to their cousins, as close as siblings. So what if they are not siblings, does it really matter? Family is family. It is only a problem if you and your parents choose to make it one. If you choose to be warm and welcoming it is no longer a problem.