You tell them that men who abuse boys put themselves in positions where they have access and preferably, authority over boys. You tell them that the abuse of older boys is abuse by a homosexual and that unless they want to assume that all homosexuals are abusers, they should perhaps back off the church and look honestly at the world around them and all the different ways that children are abused and all the different places. You tell them that most abusers of all ages and sexes are men so unless they want to assume that all men are abusers they should again, back off the church and recognize the wider problem. It doesn't mean that the church shouldn't be held accountable and the evil recognized, but there is nothing that is unique to the church except perhaps the ready access to children that priests have...just like teachers. I would also mention the deep pockets of the church and how the structure of the church leads to big lawsuits so it appears it's all the Catholic church when there just isn't the same amount of money to be made in other traditions.
I sometimes ask my anti-Catholic friends if they think there is any chance that young boys in buddhist monasteries are being abused and the answer is always "no". Except for one person who said it may be more culturally acceptable in those countries...absurd. I don't believe for a minute that there isn't abuse of boys in those places because all men and women are stained by original sin and again, men who want to abuse boys go where there are boys to abuse and homosexuals are drawn to all male environments.
I'm not ignoring the fact that many, many girls were abused in the church but the vast majority of victims were male. From wiki:
The John Jay report found that 81% of the victims were male; and of all the victims, 22% were younger than age 10, 51% were between the ages of 11 and 14, and 27% were between the ages to 15 to 17 years.
Heterosexual abusers do not target boys 11-17.