Tim:
A good question, hard to answer.
In general, genital warts are called, "venereal," even though not all of them are transmitted that way, and it's impossible to prove in most cases which ones were. I have many patients with
penileCancer - penis warts, for instance, whose long-term partners never got them, despite regular examinations.
For the speicific question you ask, I think the risk of transmission is low, though I guess no one can say it's zero.
I'm sure you're wondering: "Well, of they aren't from sex, what are warts from?" The answer is, "I don't know." Almost everybody gets warts at some point--on fingers, on the face, on the feet, etc.--yet almost never is someone in their vicinity infected. Go figure.
Best.
Dr. Rockoff