It’s a term that refers to marriages arranged by the studio, where homosexual and lesbian agree to marry to give the appearance of heterosxuality or a straight person agreed to marry the homosexual in exchange for the Hollywood perks or possibly a generous paycheque.
The modern equivalent are contracts. One clue that it is a lavender relationship or contract is that the actor and his beard get together for the length of time that it takes to produce a movie and promote and then it ends on generally good terms.
Then, when another movie is under production that actor is then dating somebody else and these relationships never culminate into marriages or any real long-term relationships and children.
So the term had its origins in Hollywood. So they’ve always promoted or encouraged perversions.
What was the relationship between Tracy and Hepburn?