I have not had this surgery, but personally I would go with what my body tells me about sex rather than what the surgeon says.
Here is some information given to patients after having a Seton put in:
"You should abstain from sexual intercourse and not put anything into your rectum or vagina (e.g. tampons) for two weeks after the procedure, as this may cause further damage to your rectum or may dislodge the seton.
After two weeks you can return to sexual intercourse if you feel comfortable but please still take care to make sure you do not dislodge the seton."