How stiff is your penis?
You don't need an erection like a rod of iron to have sex...
Even though women love a rock hard penis inside them, you don't have to be a man of steel to have penetrative sex. Low testosterone and poor blood flow are common causes of limp erections, but generally speaking, if your penis is hard enough for an erection you can have sex.
If you can roll a condom over your penis and it's stiff enough to enter your partner, you should be good to go, so don't get hung up about not having an incredibly hard dick. On the other hand, if you want to try out some penile exercises to stiffen up your erections, have a go at this:
Much like vaginal muscle exercises, Penile PC exercises can help you to maintain a harder erection during sex, and in some cases help to stave off premature ejaculation. Start out by flexing and unflexing your PC muscle 30 times. Rest for one minute, then continue for another two sets. Each time you do a set you should notice a greater control over your PC muscle due to the increased blood flow.