Ants are only attracted to peonies just before the flowers open. They dine on some of the nectar that's released by the flower as it opens. And, as it happens, the ants ultimately help protect the buds from other insects that might eat the flower. It works out well for ant and peony. The ants won't stay around when the peonies have finished blooming and won't hurt your house. Note though that peonies are not necessarily magnet plants for either hummingbirds or butterflies. I love peonies and think they have amazing fragrance, but I've never seen a butterfly or hummingbird on my plants. Honeybees love them, but don't expect them to attract much else.