Because it inherently adds a certain feminine slant to the eye (the other major thing that determines the 'femininity' of the eye is the size, women's are slightly larger on average) regardless of whether you're a male or a female. I also have longer lashes than some of my female friends. Besides, features are mix and match. Some women have broad shoulders, some men have tapering waists (another feminine feature I have), etc. These features are more associated with the biology of one sex versus another. Doesn't mean the other gender can't have them.

I read this hoping for a mention of Rarity. I didn't get one.

I am disappoint. 

Pony biological cues =/= human biological cues. She is irrelevant to the discussion.

inb4 you say Rarity is always relevant

That's a good question. It might have to do with the fact that femininity is often shown through delicacy. For example, butterflies are often viewed as feminine creature, because of their beauty and fragility. Maybe long eyelashes are considered feminine for the same basic reason, they're a sign of beauty and they themselves are delicate. 

Does that make sense? I'm just kinda spitballin' here.

Because it inherently adds a certain feminine slant to the eye (the other major thing that determines the 'femininity' of the eye is the size, women's are slightly larger on average) regardless of whether you're a male or a female. I also have longer lashes than some of my female friends. Besides, features are mix and match. Some women have broad shoulders, some men have tapering waists (another feminine feature I have), etc. These features are more associated with the biology of one sex versus another. Doesn't mean the other gender can't have them.

This seems plausible as well.

Pony biological cues =/= human biological cues. She is irrelevant to the discussion.

 

inb4 you say Rarity is always relevant

RARITY IS ALWAYS RELEVANT

I read this hoping for a mention of Rarity. I didn't get one.

 

I am disappoint. 

Lol.  I'm sorry.  Just for the record, I do love Rare.  Although I was kind of disillusioned when I found out she wears false lashes.  I mean, she's doesn't need to--she's gorgeous anyway.  But it was just...idk...disillusioning.