You're entire post was assumptions, opinions and speculation, the only factual part of your post was that 5'9 - 5'10 women are rare. Do you lack intelligence in reading comprehension or something?
They are my opinions that based on the norm.
'At 5'6", there are so many girls taller than him.' -> fact
'Many white girls are 5'7"-5'8".' -> fact
I didn't even say 5'9 - 5'10 women are rare, I said they are more rare (than 5'7'-5'8" ones). -> fact
5'11 is where it starts to get ugly for women. -> let's say it's my opinion if that makes you happy. but I believe it's pretty much the norm.
'Here's the thing, if you're 5'8" and the girl is 5'10". You're shorter, but tolerable. 2" shorter but still look good. In this case, more of the girl being too tall rather than you being too short. But if you're 5'6" and the girl is 5'10", it's a good 4"/10cm difference there.'
I showed you the difference what looks ok and what doesn't.
Just ask your friends which one looks worse. Hope that helps.