Wwyd or am I just being an idiot

[TGItsNotChristmasAnymore]

When do you celebrate your anniversary? It's mine and my bf first anniversary of our first meeting soon, we have both referred to it as our first year together but I have reason to believe he was seeing (not sure sleeping with) but definitely meeting up with another woman at this point. I also have an idea that they met/planned to meet (I think she cancelled the last time) for at least five weeks into us seeing eachother (I have worked out that at least one meeting, (the last one) was arranged after we had slept with eachother. I can't be 100% sure of this but from the things I've worked out (put together after a throw away comment by him last week) I'm quite confident I'm barking up the right tree.
The thing is I don't want to celebrate 'our' anniversary if he was seeing someone else at that point,I know he will want to mark the occasion but I don't, not on that date. How would you explain this or am I just being a twat.

We celebrate when we first met. Which is also when we first slept together. And then I was still sleeping with other people (we weren't exclusive) and I had to chase him down to see me again. But it's not entirely clear when we became exclusive so I'm not sure when else we'd mark it.
When it's the far distant past we'll probably just use it to count the years and our wedding anniversary will take precedence. Who knows?

I think if you're uncomfortable about the start date he's chosen couldn't you just say? And offer an alternative like when you moved in or said I love you or something?