Welcome back to the forum.
This is a fairly frequent occurrence in many men. The prostate gland and seminal vesicles -- the sacs in which semen accumulates, waiting for the next ejaculation -- are immediately in front of the rectum. The combination of straining plus pressure on the prostate and seminal vesicles by the fecal mass expresses semen and/or prostate gland fluids. In and of itself, it is not abnormal. However, if it is happening more frequently than before, or if it's an entirely new issue for you, it would be wise to get checked by your primary care provider or a urologist, to be sure nothing more is going on, such as prostatitis. However, this is not an STD symptom.
Below is another thread on this topic, which in turn has links to two more threads. In one of them several years ago, I wrote that this happens in "almost all men" from time to time. It's not that often -- but still, in most cases this is nothing to worry about.
http://www.medhelp.org/posts/show/859854
Regards-- HHH, MD