i think you should ask this on the sharing page thingy it's a good/informative discussion to have and more users will answer there. ( if you don't want to i could do it since i'm interested to learn too!)
For me fetishizing BL is to consistently sexualize characters, stereotype them into sexual roles. BL itself is created fetishizng homosexuality. so when fetishizing it's not boys love anymore, it turns to sex and harmful stereotypes that reflect on mlm.
For me it's the fact that heterosexual roles are applied to the characters. I mean the couples are literally categorized as either the seme or uke with the seme being all masculin and strong and a weak small uke (usually). This is not only highly fetishizing but is also a strongly inaccurate representation which is hurtful to the lgbt+ community.
I will make it simple, through differentiating kink and fetish
Kink: give you an add of sexual energy in an intercourse (such as filming, or tying yourself)
Fetish: if you "only" get arousal if you have that object in you (such as hand fetish, you can only be arouse if another people hands are beautiful in your perceptive)
So in BL, it will only be a fetish if the only way you can get arousal is through reading it and masturbating from it.
Can someone tell me what things or sayings count as fetishizing BL