If anal sex is done right there should be no pain. Going slow and lots of lube is the way to go
I am guessing you are asking about a male orgasming from anal.....So yes it is possible for a guy to orgasm from anal sex. If the prostate gland is stimulated (hazelnut sized bit of squishy flesh about 2-3" inside the anal canal) correctly ejaculation can occur. This won't contain seaman as it is the fluid from when a guy ejaculates. The prostate gland is where ejaculation fluid is stored.
It is thought that having your prostate massaged has health benefits as it is thought to reduce the risk of prostate cancer etc. You can get special toys that stimulate it and I am told it can feel really good
Hope that didn't sound too much like a science lesson but I thought it best to sound a bit intelligent