When I got home, I finally explained to mum that I was a sperm donor. Had been meaning to for ages but kept putting it off because it was likely to end up in a long involved conversation about what being a sperm donor means, what it involves, etc.
With my mum.
Hit on the basic explanation that I'm donating semen so that couples who can't have babies might be able to, which I knew mum would get since that was the situation with her and dad. She seemed to get that immediately, gave me a great big hug, then offered me a hot chocolate and biscuits...
...because that's what they give you when you donate blood!