I would not have done it. I would have written: "Thank you for your interest and thanks for putting me on notice about your marriage situation. My personal rule is to date men who are either single or divorced. I wish you luck in everything."

An analogy - many years ago I was interviewed by a NYC law firm for a summer associate position. The woman who was my main interviewer went on and on about her tobacco defense practice. Tobacco defense was hot in the 1990s. I happened to think that it was wrong to engage in tobacco defense - that the firm was on the wrong side of the road, so to speak. The firm gave me an offer. I wrote a nice thank you letter rejecting the offer. I did not write that I disapproved of the firm's involvement in and earning money from tobacco defense, because nobody asked for my opinion on that matter. They were nice enough to extend their offer, and I reciprocated with being nice enough to thank them for it and reject it in a polite way.

In your situation, you think that it is wrong of him to seek another sexual partner while remaining in a sexless marriage. Did he ask for your opinion on that matter? He did not. So your reaction was unsolicited.

I believe that even though it is a quasi-anonymous online environment, the same rules of polite and courteous behavior apply as everywhere else.