People can have more than one romantic or sexual partner at a time. This is something that works for some people and does not work for others, and takes work to decide if this is something you want to engage in or not. In order for it to be ethical, everyone involved must be aware and consent, and it must be enthusiastic consent! Ethical non monogamy is work, just like all healthy relationships require work. Polyamory, open relationships, and swinging are all possible ways to engage in ethical non monogamous relationships.