If David and Jonathan had a sexual relationship (and I'm not convinced they did), then they were both committing adultery since they were both married (David to Michal at first and Jonathan to Ahinoam). I don't think you can make an argument the Bible affirms adultery.

Of course, David ended up marrying multiple women - a clear violation of Deuteronomy 17:17. This makes it rather problematic to use David as a biblical model for marriage and sexuality.