We met at [a party] on Saturday, where we had a series of interactions I want to follow up with you about.
When we were first chatting, I really enjoyed getting to know you, and I found our conversation comfortable and fun. Later in our interactions, though, I feel that you crossed my boundaries repeatedly in a way that felt bad to me.
I'm writing you this note because I think that you are a good person who didn't mean to be invasive or boundary-crossing, and because I think you would want to know that you did so you can avoid doing it again in the future.
I liked our conversation, but I didn't want to kiss you, and I didn't like it that you didn't negotiate with me -- either verbally or physically -- before pretty forcefully going in for a kiss. When I didn't return your kiss and walked away, that should have been an indication to you that I didn't want that kind of interaction with you.
I was especially bothered that, after I left that exchange, you repeatedly approached me to put your arm around me, kiss my neck, and generally continued to try to be physically intimate with me, despite my lack of reciprocation and the fact that I kept removing myself from interactions with you.
All of this felt really bad to me, and it retroactively soured my enjoyment of our earlier conversations. While it is true that I didn't verbally say "no", I did say no through my actions, and you chose not to recognize or respect that.
It's not enough for a person not to say no -- you also need to make sure they're saying YES. You might find the concept of enthusiastic consent to be useful in understanding the difference here. This is a good article outlining the cultural setting and how seeking enthusiastic consent solves it for all of us: http://www.doctornerdlove.com/2013/03/enthusiastic-consent/. I ask that you read this before you're next in the position of moving a conversation into flirty territory.
If you have any questions about any of this, I'm open to exchanging a couple of emails to clarify as needed. It's also fine if this is all that's said on the matter. I'd also like to reiterate that I'm writing this email because I think this was a misstep, rather than intentionally predatory behavior on your part, and I think and hope that this will help you find a better interaction style for yourself in the future.