I don't think that firing someone for having an affair is appropriate (unless they are in the Church or similar). If they are TV presenters move them off Songs of Praise and other religious programmes, but whilst I am not condoning affairs, if both parties consent they are nobody's business but the families concerned.
Sexual harassment is different though. It's power play when it happens between 'the talent' (ie a high profile presenter) and someone lower down the pecking order. It doesn't matter if it's 'legal' in the sense that both parties are above the age of consent. People (usually women) should be able to go to work without fear of being treated as sex objects, and I agree with Jane that we should stop making excuses for the perpetrators - they know it's wrong.
Until we learn more about what happened in this case, which may never happen, we don't know if the BBC is being trigger-happy after the Huw Edwards case, but if something like this happened in most workplaces it would be seen as serious misconduct.