WRITTEN BY BUTTERNUT / SHRIMP SYSTEM
Some systems prefer to all date one person, whether that's another system or a singlet. When a system is collectively dating a singlet, not every alter has to be dating their partner. Every alter has different preferences, opinions and feelings, so this would make sense, especially in the case of littles (alters that are young children) because they appear as children. When a system has a partner system, they may collectively feel as though they are in a romantic relationship, even if specific alters aren't dating. Others may consider a partner system a system where most alters in one system are dating most alters in another system AKA: System A has a lot of alters that are dating alters in System B.
Sometimes, systems have multiple different partners, whether that is multiple collective partners or multiple different partners individually based on alter to alter. It is important to note that this is only okay if all partners involved are consenting. If someone is not consenting to polygamy, that is cheating.
Another thing to note is that some systems will have innersystem relationships, which means that alters within one system will date other alters within the same system. Some systems exclusively date within their own system, but not all do.