You are free to disagree with the rules, but everywhere I've checked says it was the correct decision. VAR didn't even get involved because there was no clear and obvious error.
Why was Bruno Fernandes’ goal allowed to stand against Manchester City?
This is the big question, and the explanation stands in the fact that the offside rule has recently been altered.
A player is now judged to be offside if they either touch the ball after being in an offside position, or their involvement stops an opposition player from playing the ball.
Rashford ran alongside the ball but did not touch it, and no City defenders were in a position to touch the ball and divert it away from Fernandes.
As a result, the initial on-pitch decision was overturned and the goal was awarded to Fernandes.
https://theathletic.com/4090488/2023/01/14/bruno-fernandes-goal-offside-manchester-united-city/
Why referee allowed Man United goal to stand despite Rashford offside
NBC Sports contacted the PGMOL (Professional Game Match Officials Limited), which is responsible for referees in the English professional game, and according to a report relayed by host Rebecca Lowe, the response was that:
"PGMOL have gotten back to us and have said no touch on the ball, no impact on the defender, and therefore they [VAR] could find no reason for a clear and obvious error.
"According to the PGMOL, they say [Rashford] didn't impact the defender and he didn't touch the ball and therefore Rashford was OK to do what he did and allow Fernandes to come onto the ball and score."
So the final ruling was that there was no touch on the ball by Rashford and that he also did not affect the defenders, which is why the VAR did not intervene to scratch the goal.
Rules experts are all in agreement that according to the Laws of the Game, the goal had to stand:
https://www.sportingnews.com/ca/soc...ree-rashford-offside/ac3jav7c70iprlwo3tdyo8le