I know Mei is unpopular choice for Bohannon. Naomi
wanted him to change and conform to her set of values.
He lost and killed a lot of people. He couldn't fake redemption.
Mei sees him, accepts him for who he is and what he's done.
She showed him his humanity.
Their love was much deeper.
I feel Mei's hiding her gender and reveal was an act of deep trust, not only sexual. Bohannon could trust and reveal his humanity.
She offered her honor.
He honoured her offer.
Then all night long it was honor and offer.
I actually rather liked his relationship with Mei, for all the reasons you've described.
The only problem is that, although it's definitely far superior to the horrible Naomi/Cullen match, it suffers from the comparison with the Lily/Cullen match.
As "shippers" would say, Lily/Cullen was really one of the best OTPs I've ever seen! They were perfect together. Lily was such a great character. The match had so much potential... And yet it was over before it had even begun. What a huge letdown.
So yeah, can't say I disliked Mei, but she's no Lily. Even during that really hot scene at the end, I kept thinking back on the Lily/Cullen sex scene (which, by the way, I think is one of the most beautiful sex scenes I've ever seen).