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Well the doctor was in a position of responsibility/power in relation to the people looking to get pregnant, by virtue of being the person who might facilitate a pregnancy, and he chose to abuse that power by using his own sperm instead of Donor X's, and crucially hiding that information from them. The receiving parents had no power (could not have any) in this exchange, and while I don't know what might be legal and what might not, it's certainly questionable from a moral perspective at the very least.
Is what I was trying to get at...
Yeah this is what I mean by "ick factor". I'm not sure what you mean by "power imbalance" without a bit more context.