He was most likely not homosexual. The claims of him being homosexual originate from a opinion piece published in London under a fake name which claimed that Frederick's court was a bastion of immorality with young male sex slaves being passed around.

A slightly more credible assertion is made by Voltaire, who asserted that because of Frederick's father's domineering personality that Frederick enjoyed being a passive participant in homosexual behaviour. But at the same time many German writers asserted that Frederick was a staunch heterosexual. Difficult to tell who is telling the truth.

Certainly Frederick's own personality didn't help fight the assertion that he was a homosexual. He confided to one of his father's closest ministers that he didn't find sex interesting. But at the same time he enjoyed writing lude poetry that portrayed homosexual escapades. For example one poem depicts a group of lecherous Jesuits travelling an sodomizing various people.

He also maintained a very, as one author put it, "homoerotic" court. The men of the court has no issue using homosexual language and jokes about homosexuality. And while women weren't forbidden from the court; they were clearly relegated to their own sphere. Frederick's wife was essentially put in near isolation by Frederick because he simply had no interest in interacting with her. Its problematic to try and put modern day sexual attitudes on historical figures, because they just didn't see sexual mores the same we do. But for what I can tell, Frederick's rather strict, authoritarian upbringing resulted in Frederick throwing up an emotional screen, just in general he was most happy writing poetry, or playing the flute. He never let anyone in his personal space. He was more asexual, than homosexual or heterosexual. He just wasn't interested in love, marriage, etc.

That is where the answer lies in my opinion, in Frederick's upbringing. Frederick's father was such a harsh disciplinarian and just in general the type of person who kinda had to dominate those around him emotionally and personally. Frederick's father kept him in a state of constant work. Frederick was always doing military drills, studying, etc. Frederick's father constantly abused him in public, struck him, at one point Frederick's father screamed that the Frederick should shoot himself.

To give you a further idea about Frederick's treatment by his father, let me recount the famous story of Frederick and his friend Hans Hermann von Katte. When Frederick and Von Katte tried to leave (as Frederick had reached his breaking point with regards to his fathter) Frederick's father found via a guilty courtier to informed the king of his son's attempted escape. Von Katte was sentenced to death for deserting his regiment to leave with Frederick.

on Katte was originally sentenced to death by having his limbs cut with hot irons followed by hanging. Frederick's father commuted it to beheading in front of Frederick's cell.

As Katte was being transported to his place of execution, he believed that the King would eventually show mercy. He wrote a letter to the King begging for mercy and saying he would dedicate the rest of his life to loyal service to Prussia. The letter remained unanswered. Katte eventually got permission to write his father, the last letter Katte ever sent began with these words:

I could dissolve in tears, my father, when I think that this letter will cause you the greatest sorrow that the heart of a father can feel; that your hopes for my well-being in this world and your comfort in old age must vanish for ever, [… ] that I must fall in the springtime of my years, without having borne the fruits of your efforts

Frederick meanwhile was kept in a small cell. He was given a dmin candlelight to read the Bible with, and even that was extinguished at 7 p.m. sharp. He was forced to wear rags and go through brutal interrogation. He was asked a series of questions intended to break his spirit:

179: What does he consider to be a fit punishment for his action?

180: What does a person who brings dishonour upon himself and plots desertion deserve?

183: Does he consider that he still deserves to become king?

184: Does he wish his life to be spared or not?

185: Since, in saving his life, he would ipso facto lose his honour, and, in effect, be disqualified from succeeding [to the throne], would he thus stand down in order to save his life, and renounce his right to the throne in such a manner that this could be confirmed by the entire Holy Roman Empire?

To the last question Frederick answered: "his life was not so dear to him, but His Royal Highness would surely not be so harsh in his treatment of him" Frederick was always a master at dealing with his father's extremely harsh treatment; and he did not break, which only pissed of his father further.

Katte was cheerful the night before his execution, but as the reality dawned on him, he began to feel deep sorrow and became pale. He was eventually led from his cell. He had one last encounter with Frederick:

At last, after much searching and looking about, he caught sight of his beloved [companion], His Royal Highness and Crown Prince, at the window of the castle, from whom he took leave with some courteous and friendly words spoken in French, with not a little sorrow. [After hearing the sentence read aloud and removing his jacket, wig and necktie] he knelt on the mound of sand and cried: ‘Jesus accept my spirit!’ And as he commended his soul in this manner to the hands of his Father, the redeemed head was severed from the body by a wellaimed blow from the hand and sword of the executioner Coblentz [… ]. There was nothing further to see but some quivering caused by the fresh blood and life in the body.

Frederick was forced to watch the execution, his guards were ordered to hold his face up to the bars to ensure he wouldn't miss Katte's beheading. Katte's severed head was left in full view of Frederick's cell for a couple hours.

Katte wasn't the only one to suffer either. Two of Frederick's closest friends were thrown in prison. A female of Frederick's, who was sixteen (and supposedly Frederick enjoyed flirted with her) was ordered to be violently whipped and thrown in a public jail.

So imagine growing up with someone willing to put their son through this sort of mental torture. It would condition you to maintain a very rigid appearance to the rest of the world because he was forced to do so in order to survive his father's upbringing. He despaired happiness, in the words of one historian: "he never broke down the protective screen had built in his childhood."

As a side question, how did he get "the Great" title? He was not a particularly good general like his brother. Did he realy deserve it?

Well he was certainly a good general, he was held in high regard by Napoleon who I think is a rather good judge of military skill. Not to mention Frederick wrote several pieces on warfare and won several great victories in his lifetime (Hohenfriedberg comes to mind, but Rossbach is also prominent). He took a small kingdom and beat Austria multiple times (despite Austria being infinitely more powerful).

As for why he earned the title "the Great" well this map sums it up well I think. He turned a small German kingdom into a major player in central Europe, and set the stage for Prussia to become an eventual great power after Napoleon's defeat and eventually...unify Germany. It wasn't just military achievements either, he introduced a new law code, new economic measures intended to modernize Prussia (not always successful, one criticism that could be levelled is that Frederick didn't follow more successful measures being applied in France and Britain in terms of economic growth).

My recommendations for sources relating to Frederick:

Frederick the Great by David Fraser

Frederick the Great: A life in Deeds and Letters by Giles MacDonogh

Iron Kingdom: The Rise and Fall of Prussia by Christopher Clarke

FREDERICK THE GREAT: A Military History by Dennis Showalter