I like Kate and I think William is generally a good guy, but...
For all of their own mistakes W&K were together for a number of years before they got married.
William didn't always treat Kate well during their courtship and broke up with her many times. He didn't propose until he was told to stop wasting her time (marry her or leave her) by both the Queen and Carole. When they were about 25, the newspapers went into overdrive suggesting William would be proposing soon. Photographers were camped outside her place. He didn't propose and she was essentially harassed and then the newspapers humiliated her. She asked William for full commitment in the relationship, but he split up with her instead.
He spent the famous break up (he jumped on a table and shouted "I'm free!") trying to persue Isabella Calthorpe and others, none of whom were interested enough.
Previously, his great love Jecca had refused go public too quickly and this was one of the reasons she didn't want to be part of the BRF.
When W&K got back together, William made a promise to marry her but they did not get engaged for another few years.
They spent three relatively quiet years together at university, generally ignored by the press.
They didn't know each other for the whole time. When they first started dating, William was already seeing someone.
Then Kate spent years in the public eye, gently being introduced into the Royal Family
They were hardly going to get married in their early twenties.
William insisted she be available as possible, the reason for her part time work and then no outside work. All the other royal brides began royal duties as soon as possible, again said to be William's doing.
Whilst with Kate:
A bit before Louis was conceived, William was dirty dancing with two other women in Switzerland. He was gyrating while they were 'slut dropping'.
When he was about 24 and on army leave, he didn't meet up with Kate but had many nights out. He was flirting and partying in Boujis nightclub with other women. He was drunkenly dancing on a podium with a 19-year-old in a Poole nightclub, she described the prince as, 'touchy-feely'.
William is lucky to have Kate, both now (for many reasons) and before (discreet, her pleasant and comparatively normal family and her willingness to put her life on hold for him).