Sin is sin, no matter who consents...
"We're consenting adults, so we aren't hurting anyone."
Even from a worldly standpoint, this excuse isn't logical. How many families have been torn apart by adulterous affairs between two consenting adults? How many unwanted pregnancies, abortions, latent regrets, sexually transmitted diseases, broken promises?
But suppose two adults could be involved in sinful behavior that didn't touch another person's life in any way. Would it be harmless?
Scripture says otherwise:
"Flee from sexual immorality. All other sins a person commits are outside the body, but whoever sins sexually, sins against their own body." 1 Corinthians 6:18
"Do not be deceived: God cannot be mocked. A man reaps what he sows. Whoever sows to please their flesh, from the flesh will reap destruction; whoever sows to please the Spirit, from the Spirit will reap eternal life." Galatians 6:7-8
Even if no one else is affected, sin still damages those who consent to it. There's no such thing as harmless sin.