Then when he dies we have somebody to point to and say "this person could have been saved, but for your bigotry".
FWIW surely the measure is whether you're in a risk group for transmission of disease, like being European (wrt CJD).
Also it would be "since I have homosexual sex". It's not your proclivity that's being challenged it's actions which are known to put one at greater risk of carrying certain pathogens that are detrimental to a donee.
Of course it could go down differently to how you say: He lies, the person gets a temporary reprieve and then dies from some other pathogen that the donor wasn't aware that they have but that the medics were trying to account for by ruling out those having homosexual sex.
Remember it wasn't that long ago that hundreds (thousands?) of people got AIDS from donated blood and plasma.
It's all a trade off. You trade access to the bone marrow of a certain population for an overall lower risk of disease transmission.
It's no different than the fact that if you've traveled to a malaria zone in the past year that you can't donate blood.
But often people don't react until you can point to somebody and say "because of direct consequences of this policy, this person is no longer alive". It is even better if you can show his family crying.
Personally I would prefer getting them to fix it without getting anybody killed.
But that may be too much to ask.