Honestly, it depends on the bench material, how dirty the surface is, and what you are wearing. But yes, an outdoor bench can stain your clothes after rain.
After a rain, an outdoor bench is rarely just wet. Rainwater picks up dust, pollen, tree sap, bird droppings, and whatever else has settled on the surface. As the water sits or dries, it can turn into a thin, dirty film that transfers to clothing. That is especially true for light-colored or dry-clean-only fabrics.
Some materials are bigger culprits than others. Unsealed or weathered wood is one of the most common sources of stains. Woods like cedar, redwood, and teak release natural tannins when wet. Those tannins can leave brown or yellow marks on shorts, dresses, or light pants. If the bench has been painted or stained and the finish is old, moisture can soften the surface enough to rub off. Metal benches can also cause problems if they have rust spots or chipped paint. Rust stains tend to look orange-brown and can be difficult to remove once they set. Plastic, resin, and composite benches are usually safer, but they are not completely stain-proof. If rainwater has pooled on the surface, it may carry dirt, algae, or mildew that leaves a dirty mark. Stone and concrete benches can hold moisture and grow a thin layer of algae or moss, which may leave green or dark smudges.
The best way to avoid a ruined outfit is to do a quick check before you sit. Wipe the bench with a clean paper towel or tissue. If the towel comes away dirty or colored, do not sit down unless you are willing to risk a stain. You can also look at the surface from an angle. A dry bench may still have a dull film, while a wet bench may show a rainbow-like sheen from oils or tree sap. If you want to sit, place a jacket, plastic bag, or portable seat cushion down first. Dark clothing hides many stains better than light clothing, but it is not a guarantee.
If you do sit on a damp bench and later notice a mark, act quickly. Rinse the fabric with cold water from the back side if possible. Blot the stain instead of rubbing it. For tannin or mud stains, a mild liquid laundry detergent or a paste of baking soda and water can help before washing. For rust, avoid chlorine bleach because it can set the stain. Use a rust remover made for fabrics or lemon juice and salt in sunlight, but test on a hidden area first. If the item is delicate or the stain does not improve, take it to a professional cleaner.
So yes, an outdoor bench can stain your clothes after rain, especially if it is unsealed wood, rusty metal, or covered in debris. A quick wipe test and a simple barrier are usually enough to protect your clothes.