You're right. I thought I'd posted a link already. Here's one from the BBC:
http://www.bbc.co.uk/news/health-40564329
Children and people with known medical conditions that preclude organ donation. They can (and do) use children's organs in transplants, but it's the parents or legal guardians who make that decision.
I'd never thought about that. I should have done, since I used to be a blood donor and had to stop when I started having sex with other men (as you rightly point out). I registered before then.
I'd better check the rules...
...well, it seems that I'm OK. I can't find any mention of sexual orientation on the organ donor website. Or on the blood donor website, come to that. Which is surprising, since they told me clearly that I could no longer be a blood donor. Politely, but clearly. Maybe the rules have changed in the last couple of decades. Better screening, maybe? I can find a reference to a change in 2011 which allowed gay men to donate blood after a year of celibacy but nothing else. Looks like I can start donating blood again.
Background info for anyone who thinks the timeline doesn't make sense: I'm bisexual, registered as an organ donor and blood donor at 18, didn't start having homosexual sex until my mid 20s and have been celibate for years now. Works for me - I no longer find casual sex appealing and I don't want a relationship. YMMV, each to their own and all that.