Married man wants to 'be friends'

Not necessarily anything to do w/NPD, but just a question and in need of some advice. Recently, I became friends with a guy at my gym. Nothing big. Just casual chatting. We exchanged email addresses, and I thought he was single. Many ppl at the gym do not wear wedding rings, even if married, but, for some reason I assumed he was single. Ok. We talk on instant messenger last night, and I ask about his diet. He replies…’’well, I tend to eat whatever my wife cooks.’’ Meanwhile, this man has commented that he finds me ‘’beautiful,’’ and that he is ‘’smitten.’’ These were words he used prior to mentioning his wife, so…when I found out he was married, I was taken aback. I didn’t comment though, just took it all in. This morning, I sent him an email…telling him we probably shouldn’t talk anymore. He’s married, and I didn’t mean to be flirty. He replied this afternoon, that he is the one who owes me an apology, and he went on and on…saying he was sorry if he made me uncomfortable, but he hopes that this doesn’t cause me to go away, and that I will still be open to having a friendship with him. My gut tells me, to not reply. But, then I feel rude. I dunno…what do you all think? I don’t want a friendship anymore. It just feels like we’re doing something behind his wife’s back, and a husband shouldn’t be telling another woman she’s beautiful and he’s smitten… He’s a christian man, we’ve talked about our faith. Seems like a nice person, but this cannot lead anywhere…and if I were the wife, I wouldn’t be happy about my husband talking like this…what do you all suggest? Reply or not reply to the email?