Kia's attitude changed as soon as he touched her. Youta knows what he's doing. The questionable part is why did the other guardians who know better let him touch her knowing what it's like? Her big ass cloak is what's blocking the visuals. Plus there's the small issue of her grenades that she doesn't want revealed to which is likely why she's refusing to mate. Seriously this story isn't that hard to follow so I wonder why people are forgetting key plot points like that.
Also as for him standing out in public, this isn't the first time. See chapter 45 while they were digging up the beach. It's not like he's in direct contact with the girls. As long as they don't hear his voice or touch him, they'll just think he's a rather muscular woman. See Chapter 1 when Lumi first ran into him, she didn't think anything was unusual until she heard his voice and she was even closer than these girls are. it's the knowledge of what he is along with the physical contact that sets them off, not just looking at him.
Other examples are Chapter 53 when all of the girls thanked Youta. Also Rino and Makana who were on the same boat as him for months, but didn't react to him until:
1. they saw him topless
2. He stated he was a man.
They had been in close proximity to him the entire time and didn't even think it was possible. This was long after his voice changing candy had run out. Remember the Elf in chapter 3 only gave him a one week supply (supposedly 7 candies). So they were hearing his voice for that entire time as well.