I'm not in production and I'm sure budgeting is tough, but why not book an extra model?

If you are shooting a hardcore scene, you can just make it a threeway. I know the straight companies have a "kill fee" policy that if something changes and you don't need a model, give them like $200 bucks for showing up but not performing.

I think a straight talent agency like LA Direct Models which has a good reputation for having their models show up on time and ready to work, would do well if they added some gay models to their roster