Phoenix's casual scene runs on churn. The metro adds new residents faster than almost anywhere in America, and people who just moved a thousand miles from their hometown social circle date differently: less watched, more direct, more open to figuring out what they want. Add the seasonal wave (the winter residents who arrive every October with time on their hands), a pool-centric summer culture, and one of the biggest divorced-and-restarting populations in the country, and you get a casual scene that's substantial and largely invisible: suburban on the surface, a five-million-person city underneath.
Online, overwhelmingly. Phoenix's sprawl makes bar-scene serendipity a numbers problem (the bar guide covers where the odds are best), and its overlapping suburban social circles make discretion valuable. On a purpose-built platform, both people have already said what they're looking for before anyone says hello, and nobody from the neighborhood is watching.

Adult FriendFinder is the honest answer. Default casual platform for 25+ years, with an Arizona pool that fits this city unusually well: it skews past the college crowd into the 30s-60s range where Phoenix's actual casual demand lives, it keeps things separate from your social graph, and it sorts by distance in a valley where that decides everything. Free to join and browse.
The Phoenix casual pool, sorted by distance from you, free to join and browse.
Browse Profiles FreeSet your radius tight and respect the Valley's width. Scottsdale to Surprise is a genuine expedition, and "let's meet up" dies somewhere on the 101. Ten to fifteen miles keeps it real; your side of the Valley has more people in it than most entire cities.
Set your location and see who's nearby right now, free to browse.
See Who's NearbyLooking for something more relationship-shaped instead? Start with the full dating sites comparison or the complete Phoenix singles guide.