There's something to this, but it's far from an absolute. There are rectilinear aspects to the arm (and human body as a whole), but only if you look at it from certain angles. The arm, fingers, wrist, torso, etc. are semi-cylindrical. As with any cylinder, there are some parallel or near parallel lines when viewed in silhouette or profile, but the cross-section is something else altogether - curves. It's easy to argue that curves in a wrist watch harmonize with its surroundings, rather than contrast. One can argue that the various rounded aspects of Apple Watch (corners, sides of the case, digital crown, curved sections of the side button, watch faces containing circular elements, etc.) are part of this.
Certainly the dominant shape of Apple Watch is a rectangle, but so is my grandfather's old Hamilton analog. In fact, that old Hamilton has sharper edges/corners than my Apple. While not a dominant theme, rectangular fashion watches have come and gone over the decades. Part of their appeal may lie in the contrast with the dominant, circular form. I've rarely, if ever, heard them called ugly or unattractive.
One interesting thing about those analogs is that they are more obviously rectangular (nearly a 2:1 ratio between length and width), whereas the Apple Watch is nearly square. It'd be interesting to see whether a Watch in something closer to Hamilton proportions (where the watch and band are nearly the same width) would seem more attractive to those who reject Apple's current shape.
