It's no surprise that Harumi ends up having a thing for Miriam; he's from Femille, an island ruled by a society of warrior women, which is even referred to in-universe as an "Amazon Island". Of course he's attracted to a physical powerhouse of a woman; that's what Femille regards as the ideal of beauty! Of course, it certainly helps that, in her interactions with him, Miriam is practically a Gentle Giant.
Likewise, the inferral that part of the attraction Harumi has to Miriam is because she's a Cute Monster Girl also makes sense. Firstly, if "beauty is strength" on Femille, then nonhuman races known for their strength would naturally be regarded as exotic beauties in their own right. Secondly, given how badly Harumi was treated by the human women of Femille, it makes sense he would be more attracted to nonhuman women in a world with multiple sapient humanoid species, given he lacks the traumatic past with their races.
The reason why Miriam and Harumi are Twice Shy is that both would realistically have some pretty severe insecurity hang-ups about their attractiveness. Miriam gets slapped wit both the fairly stock "nobody wants a chick with muscles" and "nobody wants a fat girl" tropes, combined with a more unique problem of having always been the village runt on an island of giants, with the added seasoning that as a fishman hybrid — and a very obvious-looking one at that — she would naturally have the Fantastic Racism angle to deal with. As for Harumi, well, he's spent his entire life as an outcast on Femille island, meaning he gets with the "no girl wants a girlish boyfriend" issue twice-over. The Wholesome Crossdresser and borderline Camp Straight traits make him too feminine for non-Femille women, and the independence, stubbornness and warrior's spirit make him too "womanly" by Femille's standards!
Harumi confesses that he has been approached as a potential recruit before... by pirate crews that wanted to use him as, to put it delicately, the ship's "courtesan".
Ramna explained to Foxy and his crew about one of the symbols of their repainted pirate flag to be Poseidon. In the world of One Piece, Poseidon is one of the Ancient Weapons from the Void Century that the World Government would kill anyone if they learned of any of their names. Hopefully, Foxy Pirates won't blab about it which might somehow reach the ears of the World Government.
