Here's the email that I sent to Mr. A earlier this evening:
Dear Mr. A,
Spending time with you again over the past couple of months has been great. I never expected it and it's been an awesome surprise. Lately - like in the past couple of weeks - I've especially appreciated that you seem much more tolerant and willing than you used to be to deal with little annoyances as they come up. Thank you for doing that. I won't forget that you've made a real effort. (And I also seriously loved meeting you at work the other day - that was awesome:)
I'm emailing you because there's something that's been bothering me for a couple days. It's important for you to know that I don't want to be involved with a man who's pursuing other women. If you're doing that now, please tell me right away.
I bring this up simply because, honestly, your conversations and texting with ["your friend Chris"] haven't been sitting right with me. But I don't want to start making up stories in my head about what's "really" going on - I would rather hear things from you.
If I'm mistakenly accusing you of doing something that you're not, I am so so sorry. So sorry. But if you are involved (or trying to get involved) with someone else, please tell me now. It would really mean the most to me for you to be honest.