Are Straight pipes bad for engine?

No. Straight's will NOT, harm you engine in any way.

Answer is correct, however:

Some of the newer vehicles use exhaust back pressure as one parameter of setting the fuel/air mixture. Additionally, having alternating cylinders "hooked together" does create a scavageing effect which enhances performance, that is why "Y" and "H" pipes are used in many dual exhaust systems (plus it sounds better to nearly everyone).