What a theory

Girls below age 20 want their close friend to be friend throughout their life.

After 20 she understand the only way a Friend can be friend throughout is getting married to him.

She starts to love the friend, as she wants him forever. They stay friends when they are lovers but romance adds with them

Here,

Lover = Friend + Romance.

When she marries her Lover he becomes Husband.When the guy becomes a husband Sex gets added to their relationship.

Husband = Lover + Sex.

So,

Husband = Friend + Romance + Sex

After few months Romance gets bored

Husband = Friend + Sex

After few years Sex gets bored

Husband = Friend

Complete the theory with the best end.

Winners will get a signed photograph of mine :-)