In the past (and still) I have a tendency to RP male characters, with female characters being few and far between, unless they're an author insert. Thus, relationships were usually rare for my characters. Though, there were more than a few homosexual relationships in their time...

In terms of viable female characters for shipping purposes, NPCs? Populate the world with NPCs. I've done that, and in MMOs, you can have relationships with NPCs. Look at all the flirting you can do in KotOR! Mass Effect has lots of NPC flirting. The Old Republic has an entire planet of NPCs for your yaoi/yuri shipping needs! Why not take a page from games and drop in some NPCs?
For more fun... they don't need to be human. Throw in a cross-breed relationship!