Gay and bi men are often excluded from donating due to higher HIV risk I believe.

For a long time (at least through the late 2010s and possibly still now, I’m no longer a good candidate to donate for other reasons), you couldn’t donate if you were a man who had sex with men (MSM) or if you had had sex with a MSM recently (6 months-2 years). Your own condom use was irrelevant.

I did once decide to stick with hands only with someone because I had an appointment to donate blood later that week. My stepsister wasn’t aware of that restriction though and I can’t imagine it’s the presiding reason why a lot of even queer women aren’t interested in bi men, given the demographics of blood donation.

There are a lot of wild things that preclude you from donating blood depending on where you live though, including time spent in the UK during the mad cow disease spike, even if you were a vegan. I understand that blood donation organizations are working with such large numbers and such a small margin for error that they would rather exclude a thousand good candidates than let one bad candidate donate, but it ends up being extremely discriminatory. I looked for some recent numbers, and it is true that even today the majority of new HIV cases occur in MSM or people who have sex with MSM, but given how widespread HIV suppression treatment is (in the US), correct condom usage reduces that risk to nearly zero.