This paper establishes what constitutes a sexual interaction between two or more people. It does this by first defining a sexual activity as one in which the agent intends to satisfy a sexual desire. To understand what it means to engage in a sexual activity with another person, it draws from Bratman’s account of shared collaborative activity. A sexual interaction is defined as one in which two or more people engage in a sexual activity together, with the intention of satisfying a sexual desire in a mutually responsive and mutually supportive way. The paper then uses this account to consider what things constitute sexual infidelity. The answer is a broad one: many things can count as sexual interactions and hence can be considered to be sexual infidelity. Contrary to what a lot of people think, this can include the private use of pornography.