yes on the first part, and the reason it's different i kind addressed in another question a couple days back, but it's kinda buried in other answers so i'll answer again

1) first like, i'm not actively telling het shippers to decenter men. most of the time i will not bring up feminism and shipping as anything more than a joke. i think calling other women "male-centered" over fanfiction is silly. it's just really hypocritical to tell fujos to decenter men when you're shipping het lol

but if we're arguing about who makes it more "about men," and people want to target fujos over it, then i do believe that m/f is actually more male-focused, because heterosexuality and patriarchy, as institutions, can't be separated.

2) shipping m/f, especially if we're talking mainstream media with canon backup for it, is still reinforcing heterosexual norms to each other. if they subvert that in any way, then it's often because that's what heterosexual women wish men were like. the subject of interest and desire is heterosexual relationships.

3) in m/m shipping, women have removed themselves from the pool of romance options for them, effectively opting out of even being vaguely associated with fictional heterosexuality. fictional "men" are involved but every aspect of it is homosocial or homosexual -- none of it is a heterosexual fantasy or pseudo-interaction.

idk how to summarize this succinctly but also irl many female groups can be used to police each other for submission to patriarchy. i am not joking when i say that the presence of heterosexuality is a risk to women's solidarity... you can look up studies on how this happens. now OBVIOUSLY i don't think that's something that happens JUST by reading m/f, but i'm mentioning that because it does create a very real difference in the two groups we're talking about.