I suspect if O'Sullivan had been offered it (perhaps he was?) he would've turned it down.
One look at the scripts would have told him his 'legit' career would be over
He and others turned the gig down, which is why the producers turned to Askwith.
Though I can't imagine those films without him really.
You say that but a fair few ordinary/mainstream actors dabbled in sex comedies during that era, though the protagonists were always relative unknowns (Barry Evans and Barry Stokes, for instance).