It's been asked a few times what exactly LMF does with the marriage AI. So, a longer answer here.

The main goal of the changes is to get the AI to narrow its selection of targets - to make it do so with better results. We want the AI to avoid certain behaviors like marrying young men to infertile women, sending princesses off to be husbands to nobodies in foreign courts, or worrying only about alliances and not at all about congenital traits or good skills.

More specifically, a list of the major alterations from vanilla:

Major

• Lots of prohibition around infertility - whether it's betrothing children to much older brides, or just marrying them to much older women in general. If it's a ruler who doesn't need more heirs, the negative still applies, though it's much lessened. There's also an age prohibition against marrying much older men, but it's far lower and can be overcome by other factors.
  
• AI rulers don't get bonuses from dynasty prestige from their own dynasty. There are also hefty bonus against marrying close (and, to a lesser extend, extended) relatives even in cases where the consanguinity rules allow it. This is to discourage families from in-marrying too much (though it'll still happen, trust me).
  
• Traits such as Witch, Sodomite, Adulterer, etc. come with hefty negatives when trying to marry them off to someone whose faith has it as shunned or illegal, unless they also have it. Unmarried women with young children much more so.

Medium

• A lot of the negatives about marrying down or marrying lowborns goes away once someone reaches the "desperation" age of 25. This is especially true for childless female rulers as they approach the age of 45. As a character gets over 25, there's a "desperation" bonus which gets larger - up to the age of 50 - and is larger for rulers and heirs who need children.
  
• Malformed characters (inbred, hunchbacked, scaly, etc.) get a big malus versus marriage with anyone. Other congenital traits will offer either a bonus or negative, depending on whether the marriage is on behalf of someone whose future they're invested in (like their close family or dynasty or themselves).
  
• Rulers and heirs will care about spouses who bring good skills to the table.

Minor

• The AI gets a much larger bonus from liking the offered bride/groom than liking their liege (unless they're the same person, of course).
  
• The AI is much more reluctant to betrothe their close family members unless there's some kind of benefit involved. Once they're an adult, that goes away.
  
• AI rulers are much more reluctant to agree to send close or extended family to marry nobodies in foreign courts. Meaning it's a marriage where the character will leave their liege's court and come to the proposer's (like a patrilinear marriage with a woman). If it's the reverse, that's less of an issue.
  
• Crusader Kings get a big bonus to marriage proposals.