There are degrees to everything. People dress and act unconventionally in public, and thank democracy that they can; the line where being unconventional crosses into giving unacceptable offence is alway hard to define, and I'm very glad I'm not a cop who has to decide it. But I would say that leading someone on a leash is pretty harmless, and would only upset viewers who had major private issues with that particular thing.

From the point of view of the BDSM community, it could even be seen as a small piece of social engineering, to educate the general public into accepting bondage fans as just plain folks. Men who had the nerve to hold hands in public in the days when it was shocking did as much as any Gay Pride march to get gays publicly accepted.