The SGA Santa is open to all genres and pairings, whether that be gen, het, slash, threesomes and moresomes, bitextual or omnisexual. While it's based around an LJ community, it's also open to those without an LJ account.
The information for signing up with an LJ account and about the ficathon is here.
The non-LJ sign up post is here.
SGA Santa open for business