I find this whole conversation fascinating!
I'm a straight male in RL and while my main avi is also male, I created a female alt long ago (primarily for messing with horny male noobs) but over time have pretty much switched exclusively to logging on as her.
Why? Well, I think female avis look much better in general and the clothes are nicer; I never did find male hair I thought looked good. I find more people are willing to interact with my female alt than my male avi. I'm not at all interested in
SL romance or cybersex so I'm not missing anything by being female instead of male. Sure I have to let the guys who IM me know that I'm totally not interested in their advances (and I say so upfront in my profile), but overall it's just more fun. If I wanted to be my regular male asshole self, I can do that perfectly fine in RL.
And when I "came out" to the original
SL friends of my male avi, most were totally accepting. In fact it's been very useful as my female friends who know my RL sex have been patient with my bizarre, poorly-thought out fashion choices and have pretty much eliminated slutgear from my inventory!