Thanks for such a great question!
I think your approach is the right one – to say something surprising that will take the conversation forward in a way that is unexpected but also constructive. I hadn’t thought of that response before about there being no sex in the age to come, but like it. It immediately indicates that however good a gift sex may be in this life (in the appropriate context), it is not ultimate: we will spend eternally happily without it. Part of the reason for this, of course, is that God has given us the gift of sexuality in order to show us something of his own love for us. It is not for nothing that God often describes himself in Scripture as a husband or groom. It is one of the primary categories for how we are to understand and relate to him. We are not just his people, but his Bride. Our human, earthly marriages are therefore a signpost to and anticipation of this ultimate heavenly union. When we have that relationship in all its perfected fulness, we will no longer need the signpost.
The other great thing about your response is that it is a general comment. It is not saying anything specifically about LGBT+ people. This is very important, I think. So often, when Christians say something specific about same-sex relationships it is (mis)heard as being an attempt to single certain people and certain sexualities out for special condemnation. I think it is often more profitable to talk about sexuality in broader categories and to show something of what the message of Jesus means for all people in this aspect of life. I often encourage Christians not to say to someone what they can’t say to everyone – in other words, to show how the gospel levels the playing field and challenges all of us before moving on to how it challenges particular people in particular ways. (I say more about that here: https://www.thegospelcoalition.org/article/hope-help-sexual-revolution/)
I hope that helps. Every blessing,