That's bc ppl have been conditioned to like the trope of:
The player, unattainable guy that only does casual sex, falls in love and finally settles down for the mc girl (or uke). The girl or uke are usually written as virgins or an average character that "aren't like other girls" but influences and sucks in the "popular male player" who is changed for the better. That's why when you see a seme sleeping around you actually like the fact that he stops for someone he loves and the change is proof of his devotion and loyalty to her/him, when before he never did that.
When the girl or uke is sleeping around it disrupts this common trope and makes people uncomfortable. This also stems from our patriarcle society and how men get away with virtually anything. Guys are seen as "player, legend etc" when he "gets the chicks" whereas women in society are seen as "sluts, whores, dirty etc" when they sleep around and "don't respect their bodies". This also disrupts the idea of the virgin character who is inexperienced and naive.
Omg what is wrong with u people , so if seme is slipping around its okey and cool but them uke is doing the same its bad?