Straight arm vs double bend on their own is not a big deal.
What is important is whether you are using pronation vs supination in the transition between the takeback and forward swing.
If you're using pronation, you tend to produce a straight arm stroke, but the fact that your arm is straight isn't what's making the big difference. In fact, you shouldn't be focusing on hitting with a straight arm at all. That comes by itself from getting proper contact position. Watch Federer and Nadal, they frequently will hit double-bend forehands if they're not in perfect position.