I think the wedding party should be comprised of your nearest and dearest (tangent: no one should feel like an obligation!). But I'm always surprised to see wedding parties fall along very strict sex lines: girls on one side, boys on the other. It's reminds me of fifth-grade recess!
Of course it makes sense to be segregated like that if all the bride's closest friends/family really are female and all the groom's are male. I'm not suggesting that you fudge your relationships in order to get more genital diversity. But honestly, people, should your fiance's sister really be on your side?
I wonder if it's partly about aesthetics. Maybe some brides worry that it won't look as good because the outfits can't be as matchy-matchy.
What are your thoughts?