Studies on muscle theory
Women universally find muscle more attractive
According to Aaron Sell, Aaron W. Lukazsweski and Michael Townsley in a blind, academic, peer-reviewed study, cues of upper body strength account for most of the variance in men's bodily attractiveness. In the study, women were asked to rate photos of men, and while height and leanness played a role, strength played the largest role in which pictures women picked, with women choosing stronger men and not weaker men. In fact, the women never chose weaker men, and there was no nuance to the results. Zero of the 160 women surveyed showed a statistical, actual preference for weaker men.
Clear sexual benefits
According to a study by UCLA, muscle-bound men report almost 3 times as many sexual partners as men without a muscular physique.
There is also allegedly greater online, self-reported appreciation for "dad bods" in America. There really is no universally accepted definition of, "Dad bod", but all definitions include a lack of abdominal muscle. This made the news to due an online study, not by any academic source. The "study" was by Kelton Global, a company which owns the gym Planet Fitness. This "unbiased study" showed a small majority of women self-reporting that "dad bods" are sexy. And a larger majority preferring to marry men with Dad Bods, than men with a six pack. If these results are true, they simply only show self-reported preference, not actual preference. In other words the women were simply asked in writing, it was in no way a blind study. Additionally, according to anecdotes, it may be true women have an actual preference for less muscular men in marriage, and this would follow the theory of the dual mating strategy.
This theory is promoted by MGTOW youtuber LFA. It is also the dominant mode of thinking on Bodybuilding.com. Those incels who are naturally not masculine in personali-tee-hee, or hate masculinity in general may avoid muscle-maxxing despite it's proven benefits and they may hyperfocus on less studied looksmaxxing, such as fashion.
William agrees with the theory and takes it a step farther. Suggesting abs is the single most important physical trait in men on online dating apps. While not as quantifiable as regular muscle theory, abs theory is hypothesized based on shirtless men with abs always getting dozens of matches on tinder during catfishing experiments.