I'm more curious as to why you're asking them if they watch porn and bat one out to it? Its not usually something that randomly comes up in conversation...
Either way, not entirely certain you have anyu business 'getting angry' at them for doing something that is both perfectly legal and harms no one.
On not caring? Stop talking to them about it i guess? At least then you wont have any reason to trigger thoughts about it. If you're constantly imagining your friends ******* to porn other than them talking about it then theres likely a deeper problem here..