"Because some men love other men, and some ladies love other ladies, just like Mummy and Daddy love each other," I replied.
My son looked thoughtful, then asked me how, if gravity is invisible, it can hold up the sun. This was a far more difficult question to answer.
We then moved on to complex negotiations about the number of biscuits he could have if he ate all his peas.
It really wasn't a big deal. So I was surprised when an aquaintance recently told me that she's terribly worried about telling her son that a close family friend is gay.
Their friend is shortly to have a civil partnership ceremony, and the whole family's been invited – including, of course, her seven-year-old, who may have questions as to the whereabouts of the bride.
"I just don't know what to tell him," she confessed. "What if it confuses him?"
I wondered if I'd confused my son by telling him the simple truth.
Two years on, he certainly enjoys wearing my blue nail varnish, while his younger brother has a penchant for carrying his soft toys around in my handbags.
However, I think this is probably because my sons are mad, rather than any incipent gayness.
Mum Julie took a similar, simple approach.
"My little boy asked me what gay meant a little while ago," she says. "He said someone at school had told him it was when two men loved each other.
"I told him that was right and that two ladies could also love each other as well. He was not phased in the slightest by it. I think honesty is always the best policy."
Perhaps it's just the shock of the new that makes some parents uncomfortable.
Nowadays, gay role models are everywhere. Same-sex relationships are regularly featured in TV programmes, films and even computer games. Pop stars, sportsmen authors and politicians are out and proud.
So it's natural and normal that children will be curious when they encounter new kinds of relationships, whether it's in the media or in their everyday lives.
But this is a relatively recent development.
Explaining homosexuality to young children wasn't an issue for my parents.
Not because they were homophobic – they weren't and still aren't. But because it simply wasn't talked about. In my world, George Michael only hadn't proposed to me yet because we hadn't met.
I didn't even know that homosexuality existed. We had no gay friends or family members – at least, if we did, it wasn't talked about.
I thought that occasionally having thoughts about snogging other girls meant I was weird and abnormal.
So I'm glad to have the opportunity to talk about being gay with my son. I'm happy that he'll never have to secretly worry the way I did.
Maybe some parents confuse 'talking about sexuality' with 'sexualisation'.
But answering a child's questions honestly and simply, whether it's about gay or straight relationships will not 'sexualise' him or her. It will not mean that others see your child in a sexual way, or that your child will become unhealthily fixated on sex.
Giving a child the facts is far more likely to help that child develop a healthy attitude to his or her own sexuality in years to come.
It'll also help your child feel that he or she can talk to you about sexual issues in the future – even if, at the moment, he's still at the stage of shouting 'YUCK!' whenever kissing is mentioned.
"It isn't like you're going to go into the sexual details at this age!" agrees mum Lisa. "Couples are together because they love each other, and that's pretty much it really."
And there's a wider benefit in talking to kids about homosexuality - helping to stop prejudice before it even starts.
Mum Kate says: "I had to have a conversation with my son recently, as some other kids at school were up before the head for calling another boy gay. I went ballistic about it when I found it was being used in the playground by way of insult.
"I only feel the need to address it when I have to flag up to my son the idiocy of some people who think it is something they need to define, ridicule or hate someone because of their sexuality."
So it wasn't hard for me to explain to my son what homosexuality is.
But I can see that it will be very hard to tell him that there are people out there who hate our neighbours simply because they love each other.
Perhaps if enough of us don't make a big deal over homosexuality, our kids won't either – and that's got to be good for all of us.
More on Parentdish: Why gay is not an insult