No, that's absolutely normal. As you get close to orgasm, muscles inside your pelvis draw the testicles close to your body (presumably to get them out of the way for intercourse). For some guys they get pulled very close. They normally don't actually go inside your body, although it can feel and look like that. (For more, read JackinQ&A – Questions About the Male Body.)
What do you think about when you masturbate or have sex? Would you tell your significant other if they asked?
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When I masturbate, my testicles sometimes seem to pop up inside my body. Does this mean anything?
