No one seems to know he is married other than me, is he really flirting though?

I am from a business family, so from childhood I was taught to know more about the people I will be in constant contact with so that it is easier to do business. I have carried this habit forever despite not being involved in business anymore. Recently I joined a training that will be helpful for my career. It will be for 4 months. There are people who have come alone like me and people who have joined with co-workers or friends too. One of these guys joined with his co-worker, but it looks like they have become friends because of this training. The training has been already going on for almost 2 months. I have also become friendly with these guys.

Because of my habit of wanting to know more about people I am in regular contact with, I did some social media digging of these guys and one of them is married. This made it more comfortable to me because both the other guy and me are in long term, happy, committed relationships. So I feel really comfortable with them. However, the topic about relationship very rarely (almost never) comes up because it is an intensive course and we only really talk about the course material and class.

Recently the married guy was absent and the other guy tells me 'Don't you see that he is trying to flirt with you?' and gives me specific examples. I was like 'What are you talking about? Isn't he married? Why would he do that?' He was like 'Nah, what are you talking about? He is single, he has been single as long as I have known him'. I didn't want to be like 'No, I have stalked you in social media, so i know', so I just said that I am in a relationship and not open to flirting or anything like that. But this guy seems to genuinely not know that the married guy is in fact married. You can just scroll his facebook and he has many pictures with his wife all over his facebook.

Things went back to normal the next day when the other guy came back to class. But it was more like the course got even more intense, so we haven't talked about anything other than coursework, not even like hellos or how are yous, etc. Today was a rare lighter day in the usually intense class and the other guy wasn't there, so it was just me and the married guy and he was talking like usual and also said some stuff what the other guy called flirting. I never took it as flirting. But today I kind of felt uncomfortable because the other guy had said it was actually flirting and not normal conversation between friends. For example grabbing coffee after class, like a lot of people grab coffee with other classmates after the class. These two guys do it too, I never took part in it because I don't drink coffee. But today him asking if i wanted to grab something to eat (because i don't drink coffee) after class felt like flirting because the other guy said so.