Flirting with friends in relationships

I have several friends/acquaintances that are in relationships. There is The Guy and Robert, Jack off Buddy and his boyfriend, the hairstylist I met with Max a while back is one of my acquaintances in a relationship.

I remember several months back the hairstylist found me on grindr. I just had a torso shot posted. We talk, I tell him who I was, then a few days later he starts sexting me nude photos of himself. I am taken back and wonder to myself, you're in a relationship, you're flirting with me, telling me how we should get together, and this is weird. It was at that time I was thinking to myself this was not alright, and I didn't know this acquaintance well enough to know if he was joking around about getting together. He would ask me all the time what I like to do with a guy, what I would do with, and then told me to meet him after his beauty shop closed for fun. I wasn't going to risk anything and just sort of ignored this guy. I had a hard time determining whether he was flirting or was serious.

The Guy and I haven't really had to worry- I am not going to do anything to get in the way of his relationship, and we aren't really flirty toward each other. Just good friends.

Then there is Jack Off Buddy. We met on adam4adam for jack off basically. He is several years older than me, but we both have a good time. I was a little weird about meeting his boyfriend/really hanging out. I remember since we had only known each other and only hung out when we were jacking off together that meeting his boyfriend at his birthday was weird. Then he insisted his boyfriend and I go to Universal Studios. The only reason it seemed weird to me was it is a guy I've done something sexual with, and now he has a boyfriend, and to me it seemed like I wonder if his boyfriend knows, and so it was weird. I know from talking about random stuff with the 2 of them they are very much in love, not wanting to cheat. The 3 of us get along well. We were able to look past the sexual thing and it hasn't come up until now. I know that Jack Off Buddy is very voyeuristic and exhibitionistic, unlike his boyfriend. Anyways, Jack Off Buddy and I text occasionally back and forth. He'll send me a funny photo or something like a picture of a children's book and ask if I was the little boy in the picture since the kid kind of did look like me. Another time he saw a book with my last name in the title, was amused, and sent it to me asking if I was related. So basically we text random pics and stuff back to each other. Now I'm conflicted I guess because he knows another side of me that goes beyond the friendship level- I am sure I'm not the only gay guy to fool around with a gay friend- so this isn't unique to me. But what about when the texts become flirty? I am sure Jack Off Buddy won't cheat, but here's what happened:

Yesterday I got a text from him and he sent me a picture of his boyfriend being felt up by airport security. He told me that another man had his hands all over his guy in the caption. I texted back, "and you're alright with that?" He said yes, as long as he gets to watch and take photos. Then he said he has a voyeuristic/exhibitionistic side and asked if I did. I said I did. He asked what I've done that was either one of those, and I told him some things. Then we started talking about fantasies. He told me he wishes his boyfriend was more exhibitionistic or voyeuristic and had given him a hand job while driving. He told me how he used to jack off for truckers as he would drive. We got to talking about have you ever done anything at the gym? He told me about getting jacked off in front of a bunch of guys at the local Ballys Fitness years ago. He started sexting me photos of his dick. He told me he has always had a dressing room fantasy and how he would like to jack off in one. He told me I should take some photos of me in one and send pics of my dick in the mirror to him. He asked what other fantasies I had and I said outdoor scenes could be hot. He said we should go to the nature preserve near his house soon and jack off. I just left it at "that would be hot." I didn't really know what to say- was he playing around- was he serious- I think it was mostly playing- but I won't get involved in between him and his boyfriend.

Does anyone else have this where they have a friend they've done something sexual with, then they are involved, yet the flirtation with you continues?