Yes, sort of.
We were only friends for three weeks before we confessed we had feelings for each other and cut off all contact. Neither of us wanted to have any kind of affair, and once we'd admitted our feelings we knew that staying friends would basically be having an emotional affair, so wouldn't be possible. We broke off all contact almost immediately. If I'm honest, I didn't want to - I wanted to be with him so left my partner straight away, but when he told his wife he had feelings for someone else she asked him to stay and work on their marriage through counselling etc and he agreed, so I didn't see or hear from him in nearly a year. He knew he wanted to be with me and so spent the year ending his marriage respectfully (i.e. did the counselling, separated, sorted finances etc). I had no idea this was happening as we had absolutely no contact and no mutual friends etc. He could have been dead for all I knew.
He got back in touch the day he and his wife agreed it was over. We met up for coffee and decided not to see each other again until his divorce was underway. Once his divorce was in progress, a month or so later, he contacted me again and within three weeks we were living together. Seven years later we're blissfully happily married and we have a beautiful little girl.
So technically we didn't have an affair as such, but his ex wife would DEFINITELY consider me to be the OW as I was the catalyst for their marriage ending, and I do feel a lot of guilt for that. I think quite a lot of people, including her, assume we were in touch / even having an affair the whole time, which we absolutely weren't. I don't really feel I have a right to correct people though because regardless of the specifics, I continued to be his friend for those three weeks when I knew I had feelings for him (and knew he had them for me, even though we hadn't admitted it to each other), and that's wrong. Looking back I don't know what I would do differently though - he is definitely my soulmate and I handled it in the best way I felt I could whilst not being a martyr and sacrificing my own happiness.
No, I never worry that he'll cheat on me. He didn't really even cheat WITH me (our friendship was definitely inappropriately emotionally close but as soon as we voiced those feelings he stepped back) and that was when faced with a really intense emotional and spiritual connection, when he was in an unfulfilling marriage (through no particular fault, they just weren't very well suited), and had no children.