There also the safety issue that if the people are selected by the family or preasures into giving they might feel preasure to be seen to be donating greater than answering screening questions truthfully.

We have seen multiple cases of vertuous people saying one thing but doing something else. Look at all those homophobic gray politicians.

If your partners volunteers you are you going to mention the same unprotected sex you had last year with affair etc. Are you really going to tell truth and have your partner react or if tell truth to blood donor service and have partner asky why you don't have needle stick wound.

This issue is well known enough that there are policies with organ donors. Ie the doctors when told the truth will "lie" to family that uncle Jim (who is secretly Hiv positive) being the wrong tissue type.

The work involved with grating a specific screened pool of blood is significant and blood service won't do it without medical reason.

Source. Had people say one thing to me when partners present and another when they are not. Have engineered situation to get partner to go get coffee while we do some examinations and then ask the questions we suspect