We would suggest using a few thoughtful statement-questions to open the conversation. So much of how you approach this will depend on the nature of your relationship. Sometimes, a frank question might seem impossible for certain relationships. In other contexts, it might be the ticket. One of the following might work:
Have you heard teachers in your church really dig into the passages that talk about sexual ethics? What do they say?
What does your church teach about the Bible? Do they believe the Bible contains errors?
If your church thinks the Bible got it wrong on God’s design plan for human sexuality, how do you know that the other teachings of Scripture are reliable?
There are a lot of ethical teachings in the Bible, can we just do away with the ones that we don’t like?