I have a lump at the top of my teste on my left testicle. It is painless. Hard like a marble. I have no other symptoms - no discharge, no dysuria, no pain, nothing. I’ve tried to squeeze out discharge but usually nothing comes out. Sometimes I’ve suspected a discharge but it would be after an erection and then the erection subsided and I would squeeze it out after maybe 10-20 minutes from my erection. 

I’ve had multiple partners unprotected, though I’ve had doxypep after each new partner. I typically see partners serially on one night stand basis. 

What could this lump be? Should I get checked? 

Epididymitis always is quite painful and the entire testicle would be enlarged and very tender to touch. It does not cause this sort of problem; no STI causes a painless testicular lump. Anybody can squeeze a small amount of fluid from the urethra if they try hard enough; I urge you to stop looking for discharge and especially to stop squeezing your penis -- that can cause inflammation itself and even discharge that doesn't mean anything. (It sounds like you are seeing a bit of pre-ejaculate fluid, i.e. "pre-cum" naturally produced during sexual arousal. Stop trying to see more.) 

You definitely need to see a physician soon about the lump, preferably a urologist. Most such problems are not serious, but testicular cancer is always a possibility in this situation. So don't take chances with it.

Given your sexual lifestyle, it would make sense to be tested for common STIs. Given your use of doxy-PEP, you can be certain you will test negative for chlamydia and syphilis. And without symptoms, probably negative for gonorrhea and HIV as well.

I hope these comments are helpful. Let me know if anything isn't clear.

HHH, MD

What are common causes of this lump? 

Could it cause a sperm blockage? 

We are not experts in all genital conditions, only STIs. I cannot say all causes of painless lumps, but the benign tumor called a fibroma and a cyst of some sort probably are common. I doubt "sperm blockage" could be either the result or the cause. See a urologist; diagnosis should be easy, based on professional exam and maybe ultrasound or other imaging.