In all genres there are clichés. In the m/m romance genre, one of these clichés is from the Japanese tradition of yaoi–the masculine, dominant Seme, and the submissive, more feminine (or boyish) Uke.
Like all tropes or clichés, there’s a reason this one exists. A lot of readers like it.
Does that make it bad?
I personally don’t think so. I know in m/m and gay romance there are many options, from both men being strong and dominant (and muscular) to both being twinks, bears, etc. There’s a lot of criticism about men being depicted in a “womanly” way in m/m, but I believe this is a separate issue. To a lot of people, the big strong top and the weaker smaller submissive is appealing. It’s the whole rescuing a damsel in distress, only this time it’s the lovely prince. There is nothing saying that a small man who may be physically weaker must therefore be feminine or womanly. He in fact could be the stronger one mentally, emotionally, socially, etc. So is all that negated by lack of physical strength? Are we that tied to physical appearance?
It’s a good question.
On top of the appeal of physically strong vs. physically weak, for those in the BDSM realm I think there is an added pleasure in seeing the big manly dominant physically outmatching the smaller submissive. (Otherwise, why would there be so much more call for male Dom/female sub? or Daddy/boy, for that matter?)
I think in the end we don’t have to run from tropes like this, but just find ways to expand within them. I’ve written stories between two equally fit, equally strong men. They’re fun.
And sometimes having one smaller and younger is fun too.