Should i avoid having sex with someone being treated with ceftin (cefuroxime) for a bacterial, periodontal infection?

Not really: Just remember that antibiotics can interfere with birth control pills.

Yes: Periodontal pathogens are passed from person to person, as are many other bacteria. The wisest thing to do would be to either wait for the antibiotic to run it's course, or to wait for that person to have their periodontal disease/bacterial infection treated. Antibiotics can also affect the action of some birth control methods as well.