My understanding is you don't want someone else's body fluids saying hello to your bloodstream. An ulcer, even if it's not bleeding, obviously increases the risk.
You could always have the recipient, presuming not a postie, wear a condom. Not as much fun, but safe. And get the ulcer looked at if you haven't already. Mouth ulcers often need antibiotics to clear because they're situated in a lousy environment for easy healing.
