I came across this question on another website and found it word for word something I was wondering myself, and as I do not want to sin, and will be heading into a marriage in a week or so, I thought I'd cut and paste the question for you to answer:
Okay, I'm embarrassed to ask this, but I'd be more embarrassed asking my priest so here it goes: What are the moral implications of penis enlargement exercises? Is it wrong in itself to want or try to become bigger? It would be wrong to do it to the point you cause yourself damage, but what if you weren't causing damage? It would be wrong to do it so that you would become the talk of the town, so to speak, but what if it was done specifically to improve sex within your marriage? A lot of it stems from insecurity. Men who have no problem with their size are wanting to be bigger because society is pushing that they're just not big enough, or that bigger is definitely better than what they are now. So let's ask if it would be okay for security reasons, like working out to become more muscular. Thoughts are welcome and appreciated.
Such "exercises" may have the outcome of immoderately and immodestly focusing on sexual matters and perhaps involve an implied rejection of God's creation, so may involve issues of the First and Sixth Commandments. Moreover, such "exercises" seem medically fruitless, except to line the pockets of the purveyors.