Hmmm… no idea what’s going on here. You’ll prbly need a good Urologist to get to the bottom (no pun intended) of this. You may need to find one who’s comfortable dealing with a patient who’s engaging in anal sex with another male - not all are, so try to get a referral from a gay friend who has had a good experience with a gay-friendly Uro. I also suggest that when you do find a good Dr., you may want to try to collect a sample of this fluid & bring it in for analysis - maybe they can determine from its makeup whether it’s urine, ejaculate, prostatic fluid or some combination.
Good luck with this unusual issue - hope you find out what’s going on. Please check back in here later & let us know what’s found - I’m very curious about it…
I answered a similar question elsewhere and stumbled on this, so I'll answer it here as well...
Yes, you had an anal orgasm, or male squirting.
During intercourse, your partner is rubbing your prostate, which gives you the urge to urinate and causes contractions in your pelvis. Your body is combining the sensation of an orgasm with the urination reflex since the prostate is pressed right up against the back of the bladder. So what’s happening is the orgasm involuntarily contracts your entire abdomen so intensely that it squeezes your prostate, your bladder, your anus, your rectum, and squeezes any liquid out of you. Anal orgasms contract more than a regular orgasm, that's why it's so "intense" (as you pout it).
Unfortunately, there is nothing you can do. Just try to enjoy the release. If you really don’t like it, then next time, when you feel it building up, ask your partner stop and take a break.
Also, and I know it's easier said than done, but really try to resist the urge to push out or contract your pelvic muscles during intercourse, this only adds fuel to the fire.
I had prostate surgery and have developed a similar problem although it is pleasurable, not painful. If I had to guess, I'd say the pressure of anal sex is pushing on something, possibly your prostate. That is how docs check the thing.