There are differences between a male entry and female entry. In ITV most of the female leads give importance to the main lead only and they don't show the female leads fall in love with others too when others are behind them so the leads are safe mostly with new males entries but the lead pair won't be safe mostly when a new female entry come because most of the times the male lead give importance to other woman too and even marry them for real or fake sometimes which happened in this show too. So new male lead is a more safe option according to me.
Third wheel will create misunderstandings and distance between the couple no matter male or female, so none is safe option.
You are right that most of the shows have female third wheel.
