What is the reason why Queen Elizabeth I did not marry or have children?

I am not certain if you are saying here that Mary Tudor (Bloody Mary) never married, but if you are saying that, you're mistaken. Mary Tudor did marry, and did try to have children. She married Philip of Spain, hoping to produce a Catholic heir to prevent the Protestant-leaning Elizabeth from succeeding her. She suffered what seems to have been a false pregnancy, but was unable to actually produce a child.

Also, let's consider the idea of "Elizabeth was really a boy who replaced the original Elizabeth" idea critically for a minute.

First of all, no one would have been executed by Henry VIII for "failing to save Princess Elizabeth" (actually Lady Elizabeth from the time of her mother's death to the time of Henry's; he had declared his marriage to her mother null and therefore she was legally illegitimate) from illness or what have you. Henry was well aware of the many ways a child (or adult) could potentially die, and would not have executed anyone for the event unless it was proven to be a result of willful negligence or malice. Other relatives of his died, including Jane Seymour, probably his most loved wife (he requested to be buried at her side when he was dying), and no one was executed for that.

Second, of all the most ridiculous, absurd things such conspirators could POSSIBLY have done, WHY would they replace the female Elizabeth with a boy? If such a switch were to take place, not even the most incredibly thick-skulled fool could possibly have considered that a good idea. Considering how very much Elizabeth physically resembled her father---and the resemblance was remarked on from the time she was very small---even the replacing her with a girl would have been likely impossible to pull off by anyone who had ever seen Henry, unless they are proposing that the conspirators substituted an otherwise unknown bastard child of Henry's of whose existence he was unaware, yet who resembled him so closely as to pass for another of his children.

Thirdly, this theory presumes that all of Elizabeth's most devoted personal servants, of whom there were quite a few, must have been in on it, or else it could not have succeeded. And none of them ever spilled the beans, or were concerned about what would have been a sure death penalty if such treason had been discovered? (Especially the maidservants and ladies in waiting who attended Elizabeth in her bedchamber, helped her dress etc. Could they have possibly kept up such a secret as this boy grew?) And that Mary, who would have loved to have an excuse to remove Elizabeth permanently from the line of succession, and had her placed under house arrest under extremely close supervision for an extended period of time, never suspected or discovered a thing. This is a patently absurd theory.

If you'd like to know more about Elizabeth, I would like to recommend Alison Weir's biography of Elizabeth I as both extremely accessible to the layperson and well-researched. And free of crackpot conspiracy theories.