How to 'counsel' in sexuality issues?

I have recently re-established contact with Christian friends in Spain. They are friends of my ultra-conservative parents. Their son is gay, their daughter lesbian, so they lament their lack of grandchildren. I am not supposed to know about their offspring’s sexuality. So in my new mostly-liberal perspective, how do I discreetly encourage my friends without causing offence to them and without shocking my elderly frail parents into a very literal heart attack?