Is this really possible?:

1) The travel dates to that undisclosed island in Spring of 1971 suggest it was possible. However, it is also possible that Margaret was already pregnant by then. Remember, this was mere weeks after a secretive wedding and over a honeymoon period in which the couple seemed very happy: https://vancouversun.com/news/local-news/this-week-in-history-1971-pm-pierre-trudeau-marries-margaret-sinclair

2) If this were the case it must have been with the full knowledge of the PM (Pierre Trudeau) - after all he would have been calling the shots on where they were traveling - not his 22 year old new wife. Also I have never met an older man who has just married a woman half his age (less in this case! - BTW the article you cited got that wrong - He was 51 not 53, she was 22) who would let her out of his sight, let alone let her .....well you get the idea! Do any of you guys out there really think this is likely?

Also this author's theory (https://medium.com/@leibowitt/of-course-fidel-castro-is-justin-trudeaus-dad-nobody-has-debunked-anything-4db6fc8a9042) is premised on Pierre Trudeau being infertile but wanting a family. If we continue down this logical path then who fathered the other 2 sons? Keep in mind all 3 boys as children, and adults look remarkably similar, and according to the argument Pierre was infertile - so where were the other 2 Cuba trips around conception dates for the other sons?

Then there is the eye color. Pierre, Margaret, and all the boys have blue eyes - only option if both parents have blue eyes since the gene for blue eyes is recessive and only displays if both genes (one from each parent) are recessive, if a dominant gene (brown) is present this is what the child's eye color will be. Castro has brown, dark brown - his parents and all known children all have brown. The chances he carries the recessive gene for blue is low given his heritage.

Even if you wanted to argue that all 3 boys were fathered by Castro, and there is no evidence (that I am aware) of further trips to Cuba between 1971 and 1976 when the other boys were born, AND that Castro does in fact carry a recessive gene for blue eyes (unlikely) the chances that all 3 boys would get that recessive trait from Castro are improbable. It wouldn't happen, brown eyes are dominant. This is basic genetics and I have similar dynamics in my own family. Green eyes and hazel are not as well understood, but blue/brown is highly predictive.

To be clear I am no fan of Trudeau - never voted for him and never would. He doesn't need to be Castro's son to be a Marxist ideologue, and we can say with certainty that the Trudeaus got along very well with Castro.