So they can target ads at you?

Ask whumpdotcom that question; he works there.

It's a starting point for building an avatar. You can change just about anything later on, including making the avatar look completely neuter (or nonhuman, or whatever), but you gotta start somewhere. Also, there are some built-in default animations (which can be overridden later) that vary by gender - walk, sit pose, that sort of thing. You can change the sex of your avatar any time you like, using the same tools that you use to modify its other physical attributes. So it's not immutable in any way, it's just a point to start from.

Edited at 2008-08-05 03:17 am (UTC)

Yeah, it'd be nice if the underlying engine could cope with "none of the above", but given that all of the default gender signifiers are mutable once you're actually in-world, I've not found it to be a real hassle in practice. And yes, I'm pretty obligatory-gender-enforcement-averse myself - initially when I signed up I was a bit concerned that I might wind up having to be more gendered in-world than I preferred. But once I started playing around with avatar options that are available, it became obvious to me pretty quickly that there's a lot of scope for gender-subversion with avatar-building. The fact that a binary m/f choice is coded into the world's OS is a bit irritating, but once one starts exploring what one can do with one's avatar, it doesn't really seem to limit one's options at all. Still, it'd probably be worthwhile to suggest a neuter basic option as a feature suggestion to Linden Labs, as a matter of principle. If someone hasn't already done that, I'll look into it.

Identification purposes? Dunno.

yeah, that's stopped me from signing up as well.