In my adult life I seem to have had a serial string of good female friends, and all but one case, no lines were ever crossed. The common denominator for my good female friends is that I was attracted to, and interested in dating, all but one of them, but I ended up being 'just friends' with them at their request.
The one exception for crossing the line, was a friend of about a year that I ended up seducing one evening after we had spent the day together. I had expressed interest in her in the past, but she always politely declined. She was crashing on my couch that night and a goodnight hug (we always hugged), turned into a kiss on the neck, a caress, more caresses, all the way to '3rd base' The next morning, things quickly escalated into oral sex and intercourse.
The relationship got really messy for about a year or so, as I had fallen in love, and she hadn't. We were were extremely sexually compatible, so even when she just wanted to return to just being friends, our sexual chemistry won out 99% of the time. Eventually after I was able to deal with my emotions, and accept that we wouldn't be a romantic couple, we settled into a FWB relationship, which continued on for many years. We joked with each other that we'd still be fucking each other when we were senior citizens!
As for guy friends? ... Uhm, no, those thoughts never crossed my mind.