Reason 1: Women enjoy witnessing a relationship wherein both partners are on an equal footing, since both are men.
I had discussed Reason #1 with someone this week who felt that BL’s actually do not portray couples on an equal footing, because they often portray inequality of income. I agreed that we often see the Rich Boy + Poor Boy trope. However, when Queer Studies Theorists discuss equality in BL's they do not mean economic but, rather, equality in domestic life and personal interactions. This is the sort of equality that we do not witness in heterosexual relationships.
For example, consider the inequality in a hetero relationship wherein the woman gets stuck cooking, cleaning, raising the children and planning all the family's social activities. Women are also expected to be the more passive partner, while it's assumed that the more aggressive man will always have the last word when making major decisions. Consider how often we hear kids ask their mothers' permission for something, only to have the mother tell their kids, "Wait until your father gets home and see what he says."
Moreover, consider how it's still regarded as the man's role to propose marriage to a woman. It's actually rather shocking that in 2023 it's still up to the man to make this vital, life altering decision, while the woman is relegated to the role of merely replying with a yes or no. Simply put, the man acts, while the woman merely reacts.
Of course, I know women who were proactive and proposed to their boyfriends first. Alas, these women were considered pushy and aggressive when their friends, family and colleagues heard about it. The unspoken rule is that assertiveness is only acceptable in a man, which is why it's only acceptable for the man to propose marriage. In a gay relationship, however, either partner can pop the question and society would not judge either of them as aggressive for doing so.
If one of the men in a gay relationship assumes the more aggressive role, then that would be the result of his individual personality simply being more aggressive than his lover's. It would not be the result of society assigning him the role of the aggressor. This is crucial, because in hetero relationships, it is society that assigns the aggressive role to the man, and the passive role to the woman.
These roles are imposed on us from outside, societal forces without our having any individual participation in the matter. With a gay couple, however, such roles are entirely determined by each man's individual personality. In short, they are each equally free to determine their own roles, whereas the woman does not have this same freedom in a hetero relationship. Society has already determined her role to be passive since birth.
When we consider all these factors, it makes a male/male relationship appear pleasantly equal. Hence, it's a top reason for why women enjoy witnessing such a relationship in BL.