[drmachen]

In this movie they talk about the husband taking his best friend with them on the honeymoon. His oldest son gets married but has trouble consummating his marriage at first. At the end we realize the oldest son is the son of the best friend who left one day suddenly early in the marriage and never came back.

[MystMoonstruck]

The Family Way (1966)
IMDb review:
A wonderful story of the human condition in working class 1960s England. The screenwriter has an almost Shakespearean feel for his characters, an understanding of these people, their way of living, and what matters to them.. .
John Mills, one of the best English screen actors ever, is incredible as the father of the groom. His character is rough and boorish and totally thoughtless when his newlywed son and bride are forced to share his family's poor flat. Dad is more concerned with memories of his long lost buddy, Billy, than he is with the lives of his own wife and family. Yet, somehow, Mills lets us feel a sensitivity behind this character that wrenches the viewers' hearts at the end of the movie.
Mills' son (Hywell Bennett) and the son's bride (Hayley Mills) are forced to live with Mom, Dad, and family in a thin-walled flat. Among other problems, the proximity of family leads the groom to a about of impotence. The thin walls have the whole family and soon the entire neighborhood aware of our hero's problem. Females tend to be sympathetic to our guy; guys tend to tease him or offer advice. One particularly sleazy character promises to get the job done for him. And, the bride seems somewhat taken by this creep. Problems naturally ensue.
Mills' daughter, Hayley, is cast in her first role as an adult, playing her real-life father's daughter-in-law. She pulls off the change from child to adult actor flawlessly.

The Family Way (1966) - IMDb

Notably, Paul McCartney wrote the score.