The predominant sexual ethic today is built on three moral principles: 1) Consent; 2) No harm involved; 3) Whatever feels good. As long as it feels good, no one is getting hurt, and those involved are consenting to it, it is deemed to be morally acceptable. Timothy Hsiao has written a great article showing why consent and harmlessness are not sufficient to justify a sexual behavior.
Regarding consent, Hsiao argues that consent ought to be based on what is good for us (not just desired by us), and thus the inherent goodness of the act – not just consent – is required. Furthermore, to give consent is to give someone moral permission to do what they would not be justified in doing absent the consent. Giving consent, then, presumes that one has the moral authority to give that permission to another. But if one lacks the moral authority to grant such permissions, consent is not sufficient to make an act ethical. If the act in question is not morally good, then the consenter lacks the proper authority to give consent.
Regarding harm, this principle is usually defined too narrowly. Harm refers to any setback to our wellbeing and flourishing. That could include our physical, spiritual, moral, psychological, and financial health. To determine if a sexual act is harmful, then, we must first determine if it is moral (because every immoral act necessarily brings harm). If we have to ask the moral question before we can determine if something is harmful, then the principle of harm cannot be what determines whether something is moral or not. At best, the principle of harm serves to confirm rather than determine a moral truth.
Check out the article.