Should your penis bend when erect?
Your penis is not meant to be absolutely rigid. If it was, sexual intercourse would be difficult. The erect penis is very flexible at the base, allowing it to be angled properly to enter the vagina during intercourse. It can also bend slightly so as to have enough "give" to prevent damage.
Of course, every erection does not achieve the same measure of hardness in the erect penis. The penis expands when blood flows into the spongy areas of the penis faster than it exits. However, the amount of blood flow varies with the strength of your sexual excitement. Hence, you can have partial erections where your penis becomes bigger, but not fully stiff.
Older men sometime have a problem achieving a full erection because of plaque clogging their arteries. This same problem leads to heart attacks, so a man having difficulty achieving a full erection for a period of time ought to see a doctor to make sure nothing serious is happening internally in his body.