Ithink what people fail to consider when judging Henry for his attitude towards his wives - his first one in particular - is the attitudes of the times and, importantly, the ages of the people when big events happened.
I don't think Henry simply decided to drop Catherine becaue he wanted Anne (regardless of whether or not Anne would only sleep with him if he made her his wife), so much as he was doing what he felt was best for both himself, his lineage, and England. In 1533 Catherine of Aragon was 48. Her only living child was 17, her last pregnancy had been sometime before. Henry knew that the only child he was going to get out of Catherine was Mary, and politically speaking having only a daughter was not a good thing.
Henry's father gained the throne through conquest and a shakey claim. There were other claimaints, and rebellions against the crown based on these claimainst occurred during both HVII and HVIII's reigns. Henry knew this, and knew that a daughter would not be seen as an acceptable heir to the throne. Previously only one woman had almost inherited the English throne, Matilda, and her attempt at doing so and the subsequent usurption of the crown plunged England into a period of time now known as the Anarchy. Henry didn't want that for his legacy; he didn't want the dynasty that his father had fought to achieve and struggled to maintain, and that he himself had struggled to maintain, simply because his wife could not give him a son (and, remember, with a 16th century understanding of the human reproductive system, the failure to produce a son was on Catherine, not Henry).
Catherine had to go not because Henry was simply some sexist pig who didn't respect women, but because at the time women were viewed as lesser and he needed a son in order to ensure the continuation of stability in England. The Pope's resistance to granting Henry an annullment wasn't because the Pope disagreed with Henry's stance, but because he was a prisoner of Holy Roman Emperor Charles V, who was Catherine's nephew. Catherine didn't want a divorce not simply for religious purposes - Henry was religious himself - but also because a divorce or annulment caused her to lose power, and she was past the age where she could feasibly marry someone else and produce heirs for them.
With Anne, I think the need for a separation was fueled more by how unpopular the Boleyns were in court and the continued lack of a son than s simple desire for a "new wife." The Boleyns upset a lot of people in their rise to power, and in doing so ultimately sowed the seeds of their own destruction. Similarly, Catherine Howard courted disaster when she entered into the affair with Thomas Culpepper, knowing full well what had happened to her cousin, Anne.
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