I'm pretty sure all of those "I love you" in there came from Nanao, Asumi never said it back to her. Asumi is treating the situation as if she's being a prostitute for Nanao, but it's obvious Nanao wants more than that. This is not a departure from the main plot, but an introduction of additional drama. These two see each other way too often for this not to have an effect on their daily interactions now that they've crossed this line.
I'd rather see these two together then Asumi and Mai, but I'm not seeing evidence that Asumi has given up on Mai for Nanao. She's just using Nanao as a sex friend. I wish it weren't so but that's where the evidence points so far.