Why? To me, it seems like he lost interest in Asuna. Otherwise, how do you explain his asexual behavior towards her? He doesn’t appreciate her new looks, doesn’t make any moves, even though they haven’t had any private moments in a long time, despite having had plenty of opportunities (from his POV). Especially in the bath, he doesn’t even look at her. On the other hand, he has very clear reactions to all the other girls.
We'll probably get some 'very convincing' statements from time to time, like 'Hmm, Asuna is behaving kinda strangely' and from Asuna, 'I have to stop this,' only for them to revert to their usual patterns of not being interested in each other. So, don't bother feeling bad
Edit: Also, at this point in the game, they’re both cheating on each other, so it doesn’t even matter who started first anymore