Mangaka most definitely didn't have to include this statement, since I've seen like fifty manga that had swimsuit trying scene without any disclaimer. It's just bit unrealistic manga trope for the sake of usually milder kind of fanservice, just like skin-thight clothes that cling to boobs to the point of showing boobs separately (I'm talking without seatbelt or bag strap).
I think it's something more like housewife and milkman/plumber, not a porn term in itself but it signifies common horny drama or porn plot. Similar thing happened to sport, especially tennis clubs in Japanese universities - they're commonly used to imply orgy clubs for normie students.