It's about sex and sex is a sin.
1 Corinthians 6:18-20 says:
"Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body. Or do you not know that your body is a temple of the Holy Spirit within you, whom you have from God? You are not your own, for you were bought with a price. So glorify God in your body."
I hope Ariana's fanbase stop enabling her and help her to see that what she is doing is immoral!