Which scenario does not constitute a sexual assault?

The scenario where both individuals consent to sex and are of legal age, not incapacitated, does not constitute a sexual assault because consent is a fundamental element in sexual activity. When both parties agree to engage in sexual relations willingly and are in a state where they can make informed and voluntary decisions, the interaction is consensual and lawful. Consent must be clear, mutual, and can be revoked at any time.

In the context of the other scenarios, physical resistance or fighting back does not determine legality but rather emphasizes a victim's response to an assault, which does not negate the act of assault itself. Previous consent does not maintain validity if the current encounter lacks mutual agreement or if one party is incapacitated. Additionally, engaging in sexual activity with someone who is incapacitated by drugs or alcohol is a clear violation of consent laws, characterizing that act as sexual assault. Therefore, the correct answer highlights the significant role of consent and mutual agreement in defining sexual encounters.