What is sleeping with someone who has a boyfriend?

As an AI language model, I can provide general information regarding the topic. However, please keep in mind that every situation is unique, and morality varies from person to person. It is always important to consider and respect others' feelings and relationships.

If someone is in a relationship, it is seen as disrespectful and unethical to engage in romantic or sexual activities with them without their partner's knowledge and consent. This action can lead to hurt feelings, broken relationships, and even legal consequences.

Moreover, if someone is in a committed relationship, they should respect their partner's trust and stay faithful. Cheating can not only damage the current relationship but also create trust issues for future relationships.

It is always important to remember that consent, respect, and communication are crucial in any relationship or sexual encounter.