This Never Usually Happens...
Common Male Insecurities
Everyone has insecurities and 'manning up' doesn't mean you have to deny your feelings: accepting yourself is far more manly. Here are some of the most common concerns.
Too small? Too big? Shower (looks big when flaccid but doesn't grow much) or grower (looks smaller but expands more when aroused)? Penis size can occupy far more space in your brain than it needs to. Porn is size-obsessed but a small penis doesn't hamper sex. In fact, some sex acts are much easier. Just make sure you get a condom that fits.
From unwanted erections when you're not in a sexual situation (or indeed, when you are) to lack of rigity when you want to play, erections can be troubling. If yours pop up at inopportune moments, tight pants can help restrain your ardour. If they're not forthcoming on a regular basis, go to your doctor to check nothing's wrong.
Penis size can occupy far more space in your brain than it needs to.
However, if you only have the occasional no show, don't panic. It could be your body's way of showing you that you're not in the right space to have sex - or just be one of life's annoying inconveniences. A respectful partner won't judge you harshly - though may need reassurance that you find them attractive.
Many men assume that the longer sex lasts, the better it is. In reality, it's about quality not quantity. Rather than thinking of unsexy things to last as long as possible, get your lover warmed up beforehand and let yourself go during sex (if you tend to come really quickly, Extended Pleasure condoms can help)