The cause of Peyronie's disease is unclear. Many researchers believe the plaque of Peyronie's disease can develop following trauma (hitting or bending) that causes localized bleeding inside the penis. However, the chance of developing Peyronie's disease from  drugs is very low and there is no absolute evidence that Peyronie's disease is related to taking beta- blockers.

So, can sex with a semi erect penis over years cause the Peyronie's Disease?

Yes most likely

Peyronie's disease is a term used to describe structural abnormalities of the tunica albuginea which include fibrosis, lumps, plaques and narrowing of the shaft of the penis. Although the cause remains unclear it most likely represents a degenerative process of human flesh and is often noted in patients with diabetes, hypercholesterolemia, endothelial dysfunction, post prostatectomy, collagen vascular disease such as Lupus and scleroderma and HIV patients (neuropathy). Because erectile functio depends on the integrity of cavernosal smooth muscle and elasticity and compliance of the tunica albuginea, erectile dysfunction may or may not be present with Peyronie's disease. Trauma can also damage the tunica albuginea, however unless the patient reports a clear history of penile fracture, most often than not it's unlikely to be the cause. 

ED caused by Pyeronies

Please take a look at this useful PDF attachment.

There is no convincing support for this notion in the literature, if any.