The previous post was very thorough. Indeed, the boys' own experiences with the notion of "happy marriages" such as their parents' have made it difficult for them to be able to accept that marriages work. There is a certain challenge present for the boys in being able to buy into the concept of marriage when their own narrative with it is replete with emotional distance and adultery within it. The fact that both sons blame their parents, specifically for much that has gone wrong in their own lives indicates that they are not in a moral or psychological place to accept the responsibilities that come along with marriage. Biff blames his own failures on the disillusion caused as a result of his father's indiscretions and becoming aware of them at an early age. In the end, Wily's own failures impacts Happy to make "right" how Wily was seen, but nowhere in this belief does one get the idea that the personal realm was a part of this. It doesn't seem as if Happy is one who is fully able to see that Wily's own failures in the personal realm is one of those areas where rectification is needed, and in this light, the same disregard for the personal is seen as it was with Wily. For these reasons, it doesn't seem like the boys are going to be predisposed to being able to "settle dow" in a conventional sense.
In addition to the previous answers, another thing to take into consideration is money. Neither Biff nor Happy have any form of financial stability to even come close to maintaining a home. Add that to their psychological immaturity and the co-dependence they have of each other as brothers. It is very hard for two young men who have been engrained the idea that money buys everything and IS everything. Biff and Willy lack the very thing that their father told them was the formula to be happy. They are as lost looking for the key to happiness as their father was. There is no room for any other goal in life when one is fixated on a specific one.
In the play "Death of a Salesman" both sons have responded to their upbringing in a home where reality is somewhat distorted and the dysfunctions of the family have affected their ability to move forward in life. Happy is the youngest son who is in a go nowhere job as a buyer's assistant in a store. He builds his position and self up by sleeping with the wives of the people who have more prestigious positions in the company.
Biff is hindered by his own high ideals for himself. He has come to believe that he is above others and does not have the same work or goals as them because he is "special. When Biff returns home he is still delusional about himself, but while waiting in the office of his friend, who ditches him, he begins to see his life in the "real" for the first time. He goes home and confronts the others with who and what they really are versus how they have lied about their lives to themselves.
Neither son is able to take on the permanence of marriage because they have seen that family values and marriage are a scam. Biff's and Happy's rememberance of their father's having cheated on their mother may have affected them making them not want to marry.