Eg if you are a 5/10 man, date a 3/10 or a 4/10 woman. Since she'll know she likely can't trade up to a better man, she'll be good to you and she's far less likely to divorce you or cheat on your ass.
If you go after women at your own level, you're going to have to work your ass off to make sure she won't want to cheat, and even then, a lot of women will STILL try to move up to better men.
If the woman you're seeing is above your level, she's almost guaranteed to cheat on your ass every time. Women who can date a 7/10 won't stay with a 5/10.
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While what you're brother is saying is somewhat true. You shouldn't not date someone if you think they're out of your league. If you both have a bond, then she cheats, she was just a bitch in the first place =\