The current "why do straight women engage with m/m relationships in media if not for fetishism?" conversation is so interesting to me because unless you have no reference to standard heterosexual relationships, at least one potential answer should be obvious.
Sociological and psychological research, medical studies, relationship satisfaction stats, and even antcedotal reports and narratives, show that the emotional burden of heterosexual relationships is largely put on the woman.
There is a gendered entitlement to care - men should receive it, women should provide it, with far less expectation of reciprocity.
Having a relationship between two people of the same gender removes this specific issue.
Why fictional m/m media, opposed to sapphic media, that could be argued to remove the same inherent gender imbalance?
Straight women, by definition, are attracted to men.
When a straight woman consumes heterosexual media the implicit expectation and interpretation is that she is attracted to the male protagonist, and is meant to live vicariously through the female protagonist. No one questions this. No one calls this 'fetishism'. No one suggests this is a moral failing. In fact, the underlying implication of this entire discussion rests on straight women being expected to only consume heterosexual media, not that they shouldn't consume any.
Fictional m/m narratives, such as the ones this conversation has been largely centred on, offer love interests of the gender that straight women are attracted to, without the burden of societal expectations and power dynamics that come with a heterosexual relationship. They want to consume romance without the burden of expected subordination due to their gender.
Of course there are many reasons women engage with m/m media, however from conversations I've had with straight women, this seems to be one of the most prevalent reasons.
I do also think it is a conversation that should remain open. There are circumstances where gay relationships are fetishised.
We, as an audience, shouldn't flatten gay mens voices or experiences.
However, I'd argue rather than only asking, "why are women obsessed with m/m relationships in media?" we should start with, "why are women disengaging with heterosexual relationships and what does that say about society?"