I have a husband and a girlfriend. I guess you could use the descriptive labels of "primary" and "secondary" but they're definitely descriptive, not prescriptive (meaning I'm not putting any limits on the "secondary" relationship that it has to remain that way, it just happens to be that way at the moment.)
My situation might be a little unique in that my husband works out of town and is only home for 3 days every other weekend. So our time together is rare and precious. I've got his weekends booked in my Google calendar until the end of time. I don't plan things on those weekends unless it's unavoidable.
My girlfriend is also married, so she completely understands the need to prioritize family over dating. She's made it clear that she doesn't like to interfere with my time with my husband, because she knows how little we get.
The only person I cancel plans for is me. I'm very introverted and a homebody. Sometimes I make plans, and when the time comes, I'm just too exhausted, or can't handle people at that moment, or just plain don't want to be out any more. Depending on which one it is, I'll tell the person and either invite them over or reschedule.
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."