My mind has now changed and I'm strongly considering getting a black Seamaster on a rubber strap. They just look fantastic. But I have very gay, small wrists and it might look ridiculous.

You should be fine.
Watches with a bezel have a smaller hour marker circle ( an imaginary circle measured from one tip of an hour marker on the dial to the tip of the opposite hour marker. )

A 40mm watch with a bezel will always look smaller than a 40mm watch with no bezel.
Here's an example : Orient Ray 2 ( case size : 41.5mm ) vs. Seiko SRPB41 ( 40,5mm )

If you are really self-conscious about your wrist size, you can always opt for an Omega Seamaster Professional 300m Mid Size ( 36,2mm ) instead of the more traditional 42mm model ( swapping the metal band for an Omega black rubber strap is very easy.)

Edit : the reviewer's wrists in the video are 6.29" ( 16 cm ) in circumference