Over the past couple of years, I have experienced multiple ejaculations. During sex, I orgasm, ejaculate and my penis remains erect and I am able to continue having sex until I ejaculate and orgasm again. I don't always ejaculate the second time, often my girlfriend has had a couple of orgasms by then and she is pretty much worn out so I stop.

I have noticed as of late, rubbery vein-like projections along the length of my penis. If I had to guess I would say it was veins with thickened-rubbery walls and not ligaments. They become very prominant and somewhat tender when I become erect.

First of all,is multiple ejaculation common?
Second of all, is it healthy or could it be damaging and the cause of my thickened-rubbery penis veins/ligaments?

Any suggestions would be greatly appreciated