This is such a great question, and an urgent one as well. I often hear people say those sorts of things too.
I think there are two things we can say in response. The first is to agree with the premise behind the statement, which is to say that we are nobody at all to tell someone else how to live. We don’t have the wisdom or the moral authority. So, I don’t presume to tell anyone how to live. But, the one person who can is God. So all we are trying to do is to pass on what he has said. We are not sharing our own perspective and wisdom, but seeking to share his. And that is very different. If we have been lovingly made by a creator God, then we would expect him to know us better than we know ourselves and to be able to show us how he means us to live. Not to do that would actually be unloving, like a parent deciding to give no instruction to a small child.
The second thing we need to stress is that what God says about our sexual conduct is deeply challenging to every single one of us. It is not that we think other people are wrong and that by implication we are doing just fine. Jesus exposes the darkness in all our hearts, when it comes to sexuality no less than any other area of life.
I keep coming back to the Sermon on the Mount. Jesus takes the Old Testament prohibition against adultery and revolutionises it: “Anyone who looks at a woman with lust has committed adultery with her in his heart” (Matt 5). He takes what was thought to apply only to external conduct and applies it to our thought life and heart. And the key point is this: your sexual integrity matters so much that it is not to be compromised, even in the privacy of someone else’s mind. When we see how precious our and others’ sexual integrity is, we realise quite how far we have fallen in compromising our own and than of other people.
So when it comes to sexuality, it is important to stress that we are all alike broken and fallen. We’re all in this together. We’re not speaking into this issue from a stance of presumed superiority. All of us need the grace and forgiveness of God. And all of us can find it
I hope that helps! Have a great day.