Adelaide was not William IV's first choice. He had proposed to a couple of other princesses who did reject him with their family's approval. William was desperate to get married and Adelaide of Saxe-Meinigen was a "spinster" princess at the age of 26. She, not wanting to remain a spinster, and the possibility of having a child who would succeed to the throne, led her to accept William. She couldn't completely foretell she would be Queen because Frederick, The Duke of York, was still alive then but childless. His wife was barren.
At 65 years of age and overweights William would not have been any woman of child bearing ages first choice.
Apparently when Princess Adelaide had the preposal of marraige put to her she locked her self away in Elizabethburg Castle and cried for several days before accepting due to lack of suiters and as a matter of duty and to allow for the smallest and poorest of german ducy of Meiningen, ruled by her brother Bernard II to benifit by the marriage to the future King of England. She did apparentley have one other suiter at the time; a german soldier who had lost a leg during the Nepoliantic wars but she wasn't too keen on this gentleman either.