When it comes to dating, there is a very distinct difference between how men and women choose mates. It's actually similar to how a lot of other animals choose mates. Men take the shotgun approach. They know exactly what they want, and they'll put forth a decent effort while shooting their shot, but will take whatever they can hit. Most men aren't super picky, especially if they don't fall in the top 20% of men as far as height, income, looks and confidence.
Women, on the other hand are passive choosers. Their strategy involves making themselves available to many men at once and then taking their pick of the best ones. Usually the men they choose outrank them in some kind of way that's significant to them. For example, an intelligent women is going to choose a guy that's smarter than her or a woman who makes $80k/yr is going to choose a guy who makes $100k/yr. Whatever she values most, the man she chooses will more than likely outdo her in that particular thing. Because of these chosen tactics, woman generally date up, and as a result, most men date down. But some women wind up date down and here are the main reasons why:
Yep, she's just plain old stupid. She has a checklist of everything she wants in a man and she thinks her current guy satisfies 90% or more of the list, but half of the list includes qualities he can easily trick her into thinking he has, like "faithfulness." Either that, or she's in denial. In the back of her mind she knows she can do better, but she tries not to think about it. Why ruin a good thing by looking for something better right?
She's just fat, ugly or has some other trait that high quality men try to avoid like kids or an attitude problem. She has plenty of options, as most women do, but her options are relatively no better than her. If she's a 3 then most of the guys that hit on her are going to be 3s or worse. And if she does manage to bag a 4+, she'll be used to cure his blue balls until he builds up the courage to talk to a 5+.
She is decent looking, if not bad, but has vulnerable self-esteem. More than likely, she was ugly/fat in the past so she grew accustomed to dating low quality guys. As her looks improved, her self-esteem remained the same. She gets hit on by high quality guys, but she feels like she's not worthy of having one so she restricts her choices to low quality men. She could also be on the rebound. She might usually date up, but right now, she just needs ANY guy to help her take her mind off her ex, even if she's out of his league.
She is a fucking unicorn. She's a 10 as far as looks, smart, makes 6 figures, has no kids, she has the full package. Problem is, there aren't any single men in her social circles that can show her up in any category. She's better than every man she meets in every way. She has no choice but to date down.
Ruined all good opportunities
To keep it simple, she's a bitch. She had plenty of chances to date up but she fuck up all her opportunities. Maybe she cheated and got caught, maybe she nagged all her exes to death or maybe she waited to long to settle down and is now past her prime. Maybe she's a 7 that feels like she deserves a 10 but all she got hit on was 8s and 9s. Now that she's lost some of her looks, she only gets hit on by 5s and 6s. Who knows. All she knows is she could've been happy with one of the guys she met/dated in the past but she ran them all off somehow.
Oh yes, this is a common one. She's pretty much just checking for niggas with big dick prints in their sweat pants. She's trying to get the bottom knocked out of her pussy before settling down and being someone's boring ass wife. She's looking for quantity; not quality. If she's drunk enough or she feels like you can fuck the shit out of her, then you got a shot.
Trying something new
So she's been dating high quality men for awhile now, and she's starting to realize that the type of men she's attracted to the most always turn out to be fuck boys. They cheat on her, lie to her, beat her, steal from her, are emotionally unavailable, never take her out on formal dates, etc. Now she wants to try dating outside her preference to see if the guys she's been passing on will treat her any better. They probably will, but unfortunately, she's clearly out of their leagues.
So, there you have it. This is why you sometimes see a bad bitch with some short, fat, ugly nigga with no fashion sense. It isn't because he's really funny or has some amazing game. More than likely, he got lucky and met the right chick at the right time. Remember, if you're close enough, a shot gun will hit exactly what you're aiming at.