To be fair, the affair is only the scandal because it may have caused a conflict of interest. Boris has had plenty of them in the past, to the point where nobody knows for certain how many children he has.
The fact that the affair has been admitted matters only because it bolsters corruption allegations.
On a side note, someone personal life choices has no impact on their work as politicians so that kind of "scandal" are not that big of a deal in France. It only increase the sales of gossip magazines.