You are correct. Sulfamethoxazole/trimethoprim is NOT affected by grapefruit.

In the case of amiodarone, atorvastatin, buspirone, and nifedipine, grapefruit juice can block the action of intestinal CYP3A4, leading to higher drug levels and a potentially greater risk of adverse effects. In the case of fexofenadine, grapefruit juice can inhibit OATP-mediated transport, leading to decreased fexofenadine levels and potentially diminished efficacy of the drug.

See the FDA's Grapefruit Juice and Some Drugs Don't Mix for more info.