For a lot of these people their (secular) religion is erasing real-world group differences. The fact that you can (whatever your sexual orientation) regularly engage in anal sex, and therefore be at a higher-risk of contracting STIs for physiological reasons, and therefore not be eligible to donate blood—and still be a good person is beyond their ability to square.
If we value your personhood equally then there must be no substantive physiological differences between you and anybody else.