because no one knows how many he's had outside his marriages and acknowledged affairs.
He has 4, aged 20 to 26. from his 2nd marriage, one from an affair in 2009 that has been acknowledged by a court, and another whose existence was alleged, but not confirmed, in a court case.
Marina threw Boris out of the marital home in 2004 over his four-year affair with journalist Petronella Wyatt, the daughter of Labour grandee Lord Wyatt. Petronella once said the politician was very proud of his Turkish heritage and his views on monogamy were decidedly Eastern. She said Boris told her: I find it genuinely unreasonable that men should be confined to one woman. Mr Johnson was sacked from his role as shadow minister by then-Tory leader Michael Howard for lying about the relationship.
Read more:
https://metro.co.uk/2019/06/22/turbulent-love-life-boris-johnson-unsettle-leadership-bid-10028790/?ito=cbshare
I don't think there was any question of Wyatt covering his political doings, so that wasn't a problem; but if Arcuri was another of his harem, and got public money, it could be a scandal.