I think there's a huge risk.and on top of that if he 'donates' prolifically and locally, then in a generation's time there will be folk unwittingly going around dating half siblings. Depends how much he 'sews them oats'but there's a risk, surely?
Genetic sexual attraction is a well-documented phenomenon, first noticed IIRC when siblings who had been separated as infants or very young children met up with each other in adult life. We are subconsciously attracted to people who are like ourselves, and it is believed that there may be an olfactory element that may mean we feel particularly comfortable with people who have a similar "smell" - in the same way that tiny babies can recognise the scent of their own birth mothers, we may retain memory of a "family" scent which makes us feel safe and protected.
If there is a large proportion of unknown siblings in a comparatively small areas, there is a likelihood that they will meet - especially if they are of similar ages and may end up at the same schools, colleges etc.
