Looking for a casual fling and threesomes with many potential candidates? Maybe you and your partner are searching for something in between. Some of them, yes. Even other platforms outside of this list are opening up to the idea of polyamorous couples and non-monogamous relationships, like OkCupid. Polyamorous relationships are just as valid and capable of being just as serious as any monogamous relationship. Ethical non-monogamy works to make an open relationship sustainable.