Why do you suppose Basil and Sybill married in the first place?

Prunella Scales explained the Sybil and Basil back-story (which she worked on with John Cleese and Connie Booth when they were all developing the character in 1975) in the 2009 documentary Fawlty Towers Revisited. Cleese also commented on how dramatically different Basil must have been when Sybil first met him.

The back-story is never directly explained in the series, but working from it helped Prunella to get a private handle on Sybil and her long and tortuous relationship with Basil.

According to Prunella: when the two characters first met, Sybil was working in her family's hotel (an established family business) on the day when Basil - a young and then seemingly dashing 'Jack the lad' fresh from his British Army national service - came for lunch at the hotel with a few friends. She was smitten with his posh voice and authoritative bearing; he was flattered by her attention, asked her out and they became a courting couple.... It was only after they were finally hitched that the young and naive Sybil actually found out what manner of man she had married!

In picturing this you have to take an imaginative leap backwards to a time when Basil was actually young, attractive and dashing; not quite so manically insane and insufferable as he is in the series! Sybil fell in love with a young man who gradually evolved over time into the snobbish, misanthropic lunatic we meet in Fawlty Towers. She genuinely didn't know what she was getting into on the day she met him, or on the day she walked up the aisle to marry him.

That's how Prunella and John saw the marriage - a youthful folly which was fated to become a bitter, middle-aged mutual pergatory for the pair of them.

I read "Fully Booked" several years ago, one of the biographies of the series.

In an interview with Pru Scales, (Sybil) she told of how before the show went to air, she paid a visit to the soon-to-be director of the show, at his home and after reading the script, asked him "Why did they get married?" The director, who was apparently ill in bed but who'd still invited Pru over, pulled a pillow over his head and sighed, "I was afraid you were going to ask me that."