I’ve seen photos of men in sashes that looks much the same as the ladies just worn off the other shoulder
Those are likely cheap costume photos you've been seeing. There are some vendors who sell absurd Scottish costumes with fake kilts, sashes, a tartan hat, and a tiny little fuzzy sporran. But there is no such thing as a men's tartan sash in traditional Highland attire. (I can't say as to whether there might be examples of men wearing tartan sashes when wearing trousers, such as for an awards ceremony or such, but it simply isn't done when wearing the kilt.)

When they make tartan sashes for faux-Scottish costumes, they are attempting to recreate the look of a full plaid, or even a belted plaid. But it is not the same thing, not by a long shot.