Great topic, Steph! I think it's a little bit of both, too. On one hand, I don't really see any of the Boys as the alpha-male "manly man" type of guy. They sing and dance, they write love songs, Brian and Howie (and sometimes Nick) dress pretty metrosexual, Brian matches his clothes to his family and wears his wife's scarves, AJ wears nail polish and makeup, and we've seen some of them in tears before. I genuinely do see them as fairly sensitive guys who are in touch with their feelings, and I like that in a guy.
On the other hand... being that we're mostly all girls, I do think we unintentionall y over-feminize them in fanfics. I know I do. I like my boys to be emo in my angsty fics and cry when it's appropriate and be romantic and confess their undying love to their ladies, and so on. I can't help but put a little bit of myself into them, and I am pretty sensitive and emotional, so I know that goes into them, too. I try to "manly them up" through dialogue and mannerisms, but I'm sure they would still come across as sort of girly to an average straight guy reading my stuff.
