A reader wanted to know if I knew of any secular arguments against multiple simultaneous sexual relationships. I don't have anything fully worked out. But I think my strategy would be first to argue that sexual relationships ought to be love relationships, with the sexual aspect being unitive.
I would then express the following worry about multiple simultaneous sexual relationships. Suppose Sally has ongoing sexual relationships with Sam and Matt. If these are not both love relationships, then they are problematic for that reason. So suppose they are both love relationships. Then I would query: What happens to Sally's union with Sam while Sally is engaging sexual relations with Matt? Either that union persists then or it does not.
If the union with Matt persists then, then Sally is not giving herself fully to Sam, and her sexual activity with Sam is not expressive to the comprehensive self-giving nature of sexual congress.
If the union with Matt does not persist, then Sally is being unfaithful to her ongoing sexually-consummated love for Matt. She has become united with Matt, but then has severed that union in favor of a union with Sam.
To make this into a full argument, one would need to argue that one ought to be faithful to love, that sexual relations are supposed to be unitive, and that sexual activity outside the context of a love relationship is morally inappropriate.