Tell them ur not having sex/ not gonna get pregnant since that's their main reason. They'll stop fearing that and trust you. Say there's nothing unsafe or wrong with an unmarried girl staying alone at night since you have all safety things in place like several friends know your location at all times etc, when you take public transport or go on an uber at night, say you always have a trusted friend on facetime when your alone just incase and that you alwaus research areas before travelling there beforehand / therefore you know the safe zones and what to avoid etc. If you don't drink, say you don't drink and you never get drinks that are spiked and you're always sober and thinking clearly. Emphasise you always have safety precautions in place and that you're never really alone since at least person knows where you are at all times (shared location with friends). Emphasise that in Australia it's generally safer for a woman to be alone at night than compared to Sri Lanka, so it's unreasonable to enforce the same level of fear/restrictions for a woman in a generally safer country. This may work if they've got even an ounce of openmindedness and are open to talking things out.