Just a preference, I suppose. It's creepy to me to be romantically involved with someone who keeps that big of a part of their life separated from me. I'm not saying that I'd need to meet the SO as a condition of beginning in a relationship, but I would at least need to know that meeting them was a goal eventually. Otherwise I would never feel welcome in their home.
Projecting myself into a poly scenario in which my lovers had other lovers for a minute...
...If I were in that situation, I just would not be able to participate in a DADT relationship.
If nothing else, my insatiable curiosity would probably drive me to surreptitious means of finding out more information about my SO's other partner/love interests/fwb. And that would undoubtedly be a dealbreaker from their pov, in turn.
I simply have an intense need to know about people and situations that have the potential to impact my life, my relationships, and my health.
Not that I'd necessarily need, or want(!) to know the down-and-dirty details, but I would require the basics be communicated to me at least, such as the OSO's name and personal details, living situation (age, gender, other partners, children?) and intentions regarding future expectations, safe sex, and the like.
At some point, I would prefer to have some form of communication with the other party, be that casual/occasional conversations over phone or social media or real life meetings.
However IF either of my partners were intimately involved with another person, I don't think I'd want to be faced with the reality of that for an extended period of time in real life.