Because having sex with your first cousin is weird and gross. William was a sexually deviant usurper who murdered huge numbers of innocent people. It wouldn't surprise me at all if William was a psychopath or sociopath, or a narcissist at the very least.
You're trying to import modern day morality into the 17th century. People in the 17th century did plenty of things that would make us nauseous. You can't apply modern values to someone who has been dead 300+ years.
Trying to diagnose a historical figure with a psychological or personality order is frankly a little odd, and insulting to people to have those actual issues. I work in mental health and work closely with people with such issues every single day. William of Orange they are not, and it's an insulting comparison to make. I get you don't like the guy, but don't do that: that's what is weird and gross. There are people out there that suffer with those real issues. Don't make light of it to try and score points on an internet forum.
James VI & I didn't marry his first cousin. Charles I didn't marry his first cousin. Charles II didn't marry his first cousin. James VII & II didn't marry his first cousin. James VIII & III didn't marry his first cousin. Charles III didn't marry his first cousin, Henry I & IX didn't marry at all. Anne didn't marry her first cousin. Mary was the only Stuart who did.
Marrying a third cousin or more distant isn't really a big deal, since third cousins share less than 1% of their DNA.
Let's look at thepeerage.com, they have a consanguinity index for every royal. (This shows how inbred they are)
Again, not sure of the obsession with genetics.
James VI married a Protestant princess because Scotland had broken away from Catholicism. Denmark was not a strange place to look for a bride, and previous Danish princesses had become Queens of Scotland.
Charles I married a French Bourbon Princess: the new French dynasty. Stuart marriages into France were not uncommon, though.
Charles II married a Portuguese Princess, a bit of an odd match, but his Restoration meant that his options were limited. During his exile, he was for a time considered as a husband for a French cousin (Le Grande Madamoiselle) and more distant cousins, such as Elisabeth of Bohemia's daughters.
James II's options were pretty limited in exile. His marriage with Anne Hyde was largely viewed a mesalliance. He wanted out of it, but Charles II ordered the marriage to stand. After she died, his options were limited as a second son. Mary of Modena was definitely a more afield match, but he wanted a Catholic bride.
Mary was married for William because of political reasons: primarily
because of political reasons. You act like she had a choice in matter (she absolutely didn't). Charles II arranged the marriage because of discontent over James II's remarriage, iirc. It was done to appease the English public.
Here's some Hanoverians;
George II (parents were first cousins) - 7.32%
Princess Charlotte (daughter of George IV, parents were first cousins) - 9.1%
Edward VII (parents were first cousins) - 8.15%
Compare to the legitimate Stuarts:
Charles I - 0.65%
Charles II and James VII & II - 0.61%
James VIII & III ("the Old Pretender") - 0.37%
Charles III ("Bonnie Prince Charlie") and Henry I & IX ("Cardinal Duke of York") - 0.45%
Queen Anne and Mary II - 0% (King James and Anne Hyde were not related at all)
Queen Anne's son Prince William - 1.51% (Anne and her husband George were distantly related, but not close enough to make it weird or cause genetic problems)
This information is useless and means nothing. Do you think royal families were considering closeness when arranging marriages? They weren't. The Hannoverians were a German dynasty, and Protestant: it's no surprise they sought out brides in Germany, and were more closely related. They also wed post-Glorious Revolution, when the crown was neutered and even
marrying a Catholic could disqualify someone from the succession. The Stuarts had more freedom to arrange marriages: especially cross-religious marriages.
I mean, James VII & II did marry Anne Hyde, and she wasn't even noble at the time. Certainly the natively British Stuarts seemed more interested in British women than the German Hanoverians did.
He knocked her up and basically was forced to marry her. As soon as he could, your savior James II tried to throw her under the bus: he claimed the child wasn't his, she'd slept with numerous men, ect. He wanted a divorce because of his poor choices. Charles II said sorry, you're staying married.
The Stuarts had more genetic variety because the crown had more power. They could marry Catholics (despite the discontent). The Hannoverians didn't have that option. Germany became the hunting ground for consorts, and continued until Queen Victoria.