I don't really like the term secondary, but I'm not sure peripheral is any better. In fact, I don't like labels at all, I struggle with them...
Using such labels is one way to help differentiate those mates you live with from those you don't and can be helpful when explaining things to others but I find the term secondary sort of demeaning and I don't like to think of myself (or my boyfriend) as 'secondary'.
Such labels don't describe my feelings at all...my boyfriend does not feel like a secondary love. I love my husband AND I love my boyfriend. Logistics and life's circumstances do come in the way of being able to live together as mates, but it sure isn't for lack of love or want.
I tried being reasonable. I didn't like it. ~Clint Eastwood~