The ejaculation process as I have experienced it and read about it, goes like this.......
1. During arousal, the bladder and outer sphincters close firmly (why we can't pee when we are hard) and the prostate chamber preloads with seminal fluid composed of proteins, proteolytic enzymes, prostatic acid, phosphatase, prostate-specific antigen and zinc. This serves as a ph regulator for the penis and the interior of the vagina and as a nourishing vehicle for living semen. At the same time, the scrotum contracts and pressurizes the vas deferens with semen, some of which may or may not enter the prostate chamber. Also at the same time, the Cowpers glands outside the prostate produce clear or cloudy viscous "precum" which fill the penis and lubricate the opening, aiding in the first penetration.
2. When orgasm/ejaculation commences, the first (preseminal) contraction empties the prostate of seminal fluid, forcing it through the outer sphincter (pleasure sensation) into the penis and, sometimes, out of the penis.
3. The depressurized prostate chamber reloads with a mix of semen forced through one-way ducts at the ends of the vas deferens, and more seminal fluid. When it is adequately loaded, it contracts again, expelling its load of live semen and seminal fluid, repeating the process until the prostate doesn't pressurize enough to induce another contraction.
If there is an object through the outer sphincter, prostate and bladder sphincter, the whole process is impaired. The sphincters can't restrain urine or premature draining of the prostate except by incidental blocking of the openings and the prostate chamber can't fill normally even though the full sensation induces contractions. While these processes aren't completely prevented, I would think delicate tissues would be stressed and even damaged by contracting with a foreign object present, possibly permanently impairing the ejaculatory function.
This is why sound to (what I believe is) the outer sphincter and extract it ahead of the preseminal contraction. I get the benefit of intense stimulation that accelerates the speed and volume of the preseminal and seminal contractions but gets out it of the way when ejaculation starts. Since I don't want to risk a diminishing the enjoyment of heterosexual sex, I do not let contractions occur with a sound inserted beyond the outer sphincter.