It is not sperm you are seeing. That is prostatic fluid. Your urethra
leaves your bladder and passes through your prostate
before going on to your penis. During sex, prostatic fluid is mixed with sperm and enters your urethra at the prostate.
The most obvious reason for passing prostatic fluid while trying to void your bowels is that when you bear down, your stool is pressing against the wall of the intestines right at the prostate. Most likely, this means that you are a little dehydrated and your stools are firmer than normal (slight constipation
), or your are having some issue with your prostate like infection or enlargement.
I would not think that anything you mentioned in your post is to be alarmed about, but you should speak to your physician just so that they can rule out anything more serious and help you relieve whatever is going on.