Because he gets to have both worlds with no compromise. Since she always went with the flow and let those lines blur he got more comfort in his comfort zone. I don’t think he purposely made it happen but it help him me okay with maybe never crossing the relationship line.
He gets to be friends and have that security of him being her priority and getting to shuu away other potentials.

How does that even make sense? I get that you’re super paranoid from PTSD but it’s like, why would you even sleep with her and cross that line that you’re so scared to cross?!?!?! I hope this actually wakes you up (╬ ̄皿 ̄)凸