Houston's casual scene is enormous for three structural reasons. The city is the fourth-largest in America. Its economy runs on shifts: the Medical Center, the refineries, the port, the restaurants; someone is always getting off work at midnight with a free morning ahead. And it's the most diverse major city in the country, which means a dating pool with every culture, every scene, and every definition of what people are looking for. Like Dallas, though, the scene runs discreetly: Texas manners on the surface, a very big city underneath.
Online, overwhelmingly. Houston's sprawl makes bar-scene serendipity a numbers problem (the bar guide covers where the odds are best), and its overlapping social and professional 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 work is watching.

Adult FriendFinder is the honest answer. Default casual platform for 25+ years, Houston is one of its biggest markets, and it solves the two local problems at once: discretion (profiles live apart from your social graph) and distance (it sorts by proximity in a metro 60 miles wide). Free to join and browse.
The Houston casual pool, sorted by distance from you, free to join and browse.
Browse Profiles FreeSet your radius tight and respect the Loop. Inside-610 to Katy is a genuine expedition on a weeknight, and "let's meet up" dies somewhere on I-10. Ten to fifteen miles keeps it real; your side of Houston 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 Houston singles guide.