For years I rode a 58 cm frame when my ideal is 54, though I wasn't actually touching the top tube when straddling it. If you're actually touching the top tube on flat ground in cycling shoes, then I think riding this frame is a bit risky. If shoes give you a smidge of clearance, you can probably ride it with proper caution. Note that you give yourself more clearance by leaning the bike while stopped at a stop light. But the main concern is stopping on hills, because the bike will be parallel to the ground and you'll be perpendicular to gravity, noticeably decreasing your clearance. So go test your clearance on hills to make the most informed decision.