The dose of radiation someone else could get from being near the seeds will depend on the proxity to the source of the radiation and the amount of cumulative time spent at close proximity.
Without wishing to be indelicate, if a man is receiving anal penetration, his partner is a lot, lot closer to his prostate than if he's the one doing the penetrating!
The type of radiation released by the seeds will only travel a very short distance, only negligible amounts will be detectable outside the man's body.
A child or pet could potentially spend a lot more time on or near someone's lap than an adult having sex would do!
Radiation exposure inutero when a foetus is developing has potentially more serious consequences.
The actual risk in any of these scenarios is small but hard to quantify and so avoiding exposure is the most obvious advice to give.