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Semen is a pretty interesting thing. In the moments before ejaculation, four substances meet at the base of the penis: sperm from the testicles plus secretions from the prostate, seminal vesicle, and bulbourethral gland. After expulsion, they emerge as a thick whitish-gray fluid. It is normal for semen to be thick initially. It usually liquefies after 30 minutes.
So thick semen is not a problem. If this is normal for your partner, I wouldn't worry. If it is new (and lasts more than 1 or 2 weeks) or he has pain in his testicles, stomach, or prostate area, a visit to a health care provider might be a good idea.