Clint Launceston, 25
Vet and international cyclist
"If a woman donates an egg, she's a hero. If a man donates sperm, he's the butt of jokes. Yet it's a vital way for some people to have a family. The image of sperm donation does need humanising and should be viewed the same as blood donation. I've got a medical background so I see science as a force for good.
"If you pay donors then you'd get more men signing up but it may attract a different type of donor. Money wouldn't have made a difference to me. For me, it was about doing the right thing and there's also something reassuring about my genes carrying on if I get run over by a bus tomorrow.
"I'm curious to know (if I have children) but it's not a burning need. I don't think I'd see them as my children. I wouldn't know how their minds work because I'm not their parent. Biologically I'm the father but I've not put in the effort raising them. And that's what parenting is about. I was brought up just by my mother from the age of five. I look and sound like my father but I'm the product of the woman who raised me. But I'd be very willing and perfectly comfortable to meet up. When I was 21, my father told me that I had an older sibling from another relationship and it didn't bother me. I thought it was fantastic.
"My girlfriend knows I've donated and 95 per cent of my friends have been positive. The fact is you're not changing society as we know it. You're mainly helping married couples have children. Isn't that what society approves of?
"Sperm donation can put a huge strain on your relationship, especially as you have to abstain from sex for about three days before you give your sample. It takes nearly a year to donate because there's a lot of screening and tests, especially psychological ones. You get tested for genetic diseases as well so there's a twinge of apprehension there.
"You get asked if you want to write your children a letter and I jumped at the chance. Mine was along the lines of: 'Hi. I'm the person who helped your parents make you. If Facebook still exists when you're 18 then look me up'."
Peter Law, 28
Musician and producer from south London
"Most of my friends are married or divorced or have children. When I visit my friends and see kids running around I definitely want children but just not yet. I'm single, I'm still finishing my degree and I want to travel. My passion is music - I play the guitar in a band, I teach music and produce - so it's hard getting a break and establishing yourself.
"I suppose donating and having my sperm frozen for my own use later is an insurance policy. So I Googled clinics which stored sperm. Then I thought, 'why not donate as well?'
"You're doing something good for others. I had a perfect childhood, then my parents divorced but I'm still close to them both and my sister Louise. The family unit is important to me - it's about love and strength. I don't see sperm donation as creating children then giving them away. I'd be the father although someone else is bringing them up.
"If someone asks me if I've donated then I'm not going to lie. I do have this close female friend and I wasn't going to tell her but I had to abstain from sex before donating - as a man you can't pretend to a woman that 'I'm on my period'. I think I'd tell any future partners that I could have donor children and I wouldn't choose to be with someone who isn't open-minded. My mum and dad, my sister know - they're all quite open-minded anyway.
"Sperm can be stored for 50 years so I could be a pensioner before my children trace me. It would make me really happy though that someone had made the effort to find me. Of course I'll think about the children I've created. I'm going to write them a letter. I want to write in case the kid is curious. It's hard, though, because I don't know where I'll be when they reach 18, what planet I'll be on or even if I'll be alive.
"To me, giving sperm is like donating a kidney - if something in your body doesn't work then you use someone else's. I do realise, though, that I'm making a decision now that has consequences later in life. But I don't think I'm irresponsible being a donor. Having children by accident, now that's irresponsible."
Joe Turner, 38
Married software engineer from west London
"My wife Sam has never had any problems about me donating. She suggested it after reading an article about the shortage of donors. Perhaps if I'd done it secretly then she'd have been upset, especially if a strange teenager knocked at the door and said 'you're my daddy.'
"We've got two children of our own aged three and seven but when you can't have any it must be hard. The shortage of donors surprised me so I thought if I could help then I would. The fact that I'm adopted was another major factor in my decision. If anyone knows what it's like to grow up without biological relatives then meet them, I do. My birth mother contacted me a few years ago after being forced to give me up as a teenager. It's all been good and I keep in touch with her and my half-sisters. Being adopted was never a problem but I was told early on, and that's really important with sperm donor children as well.
"It's a human right to know about your biology - I feel very strongly about that. I'm very proud to have donated and I always tell people. The first thing they ask is 'Why? You're giving away your children'. Some react with horror when I tell them my donor children can contact me at age 18. A lot comes down to ignorance - people's knee-jerk reaction.
"The fact is that IVF exists, you can't uninvent it. Women in heterosexual or lesbian relationships and single women need donors. I trust the bank not to give my sperm to someone incapable of bringing up a child. The alternative is women using companies selling live sperm - that's disgusting. They go through the internet but the sperm hasn't been tested for disease. Donating through a licensed clinic is a rigorous process. Only a tiny percentage of men get accepted.
"It would be interesting to meet my children but they're not my family. It's not that I don't feel any emotion but 'biological father' is just a medical term. Having said that, if one of them felt differently that's fine. And I'd also respect it if they didn't want to contact me.
"I haven't rehearsed what I'd say if my child came knocking. What I do know, though, is I'm the perfect person to answer their questions."
The pictured donors' names have all been changed.