well if it helps, my STBXH defined his physical affair as 'beginning' with an 'emotional relationship'. What that meant in practice was that he a) lied about seeing her, speaking to her etc. b) he prioritised his time with her over me and our DS to the point of excluding us from activities where she would be so that he could freely be closer than was appropriate with her even before they had any physical relationship. c) he refused to introduce her to me, but arranged time with her that included our son so they could play act at being a couple, with my DS thrown into the mix (at the time aged around 15mo).
The reason it was an affair and not a friendship even at that point was the fact that his prior relationship (we were married) was essentially destroyed to make space for theirs to develop. Oh, and also, as soon as the opportunity presented itself, they moved on to a sexual relationship - which was justified in his eyes because our marriage was 'essentially over' (nice of him to tell me hey?). He engineered the total lack of sexual and emotional intimacy in our marriage, so that he could be with her. By the time they did have sex and I discovered they were having a full blown physical affair, he was so far gone from our marriage he basically didn't even consider himself married!! I would say the detachment from me took roughly a 10 months. Six of those he was actively 'in a relationship' (his own words) with her, that wasn't physical. 4 months of that they had moved onto having sex behind my back. All this happened despite me trying to arrange relationship counselling, booking a romantic weekend away to reconnect (which he spent the majority of texting her and ignoring me), and culminated with me falling pregnant (the cherry on the cake) which he explained away by saying 'if I had been fully awake and realised who I was with it would never have happened'.
Now there's a story to tell your DC. Conceived purely because his dad was fantasising about fucking his 'friend' instead of mummy... classy. Thanks for that STBXH. Thanks a lot. Incidentally, I thought we were having make up sex due to him treating me appallingly all week - and this happened when he knew full well I wasn't on contraception because we'd had a talk about planning another child 5 months previously. Obviously that slipped his mind too, given that so much of his emotional energy was taken up with pretending he was in a relationship with her, and not me. It also explains how he could be so unbelievably cold as to tell me to 'make my own moral decision' about what to do when it became clear I was likely to be pregnant, and he was likely to leave. Having an emotional relationship with her allowed him to turn me, his wife, into an inconvenient non-person instead.