We're in the same boat, and definitely do no view ourselves as swingers. We started exploring our thinking at a swingers board, and found that the completely sexual focus left us a little cold.
For us, we view our marriage of 15 years as the primary relationship in our lives, and with all the shared experiences and love involved, nothing could ever replace it.
We're not looking to add anyone to our family, or find another 'spouse', but we feel that we want to allow our relationships with others to grow and expand without the limitations normally applied. If that means good friends with someone of either gender, great (which we have anyway), but if it means that we want to express those relationships sexually, that's great too. We don't want to limit our love, or repress our emotions, or deny the natural expressions of love we may have for other people.
And, to be completely honest, we're also curious what it might be like to have sex with someone else. We're just not entirely comfortable doing so in a sleazy, promiscuous way . . .