Mostly related, I think:
JM genuinely believes they had sex that day. IMO he doesn't base that belief just on the fact of date and time stamped photos being recovered from TA's camera. I don't think there is more evidence of sex that day that wasn't introduced, I just think JM reached his own conclusions about who Travis was, and TA's vulnerabilities.
And that's where I think there is an opening, however slight, for doubt about sex that day, in addition to thinking sex that day is counter intuitive at best, impossible at worse.
JM is unapologetic about what he focused on in the case, which was he put it, about "a gun, a knife, a brutal slaughter, and the need to prove premeditation." That's it. Perfect for trying the case, successfully and brilliantly, but not so much for the unrelated and discretionary task of trying to figure out all that the did to torment and control Travis.
It's always been telling to me that JM didn't think to question the legitimacy of all her texts and emails to TA (and of the "May 10" sex call) until well into trial one, if then. As Nurmi said, the State could have chosen to examine her Helio at any time after it was "found," but chose not to. (JM did pick up on several of the 's "accidental" texts; her habit of sending those was easy to identify from even a quick scan of the text records).
IMO JM didn't fully explore the 's manipulations because doing so wasn't a priority, and because they were, in his estimation, either irrelevant, inconsequential, or inadmissable evidence even if they could be proven.
His line of thinking also brought him to the conclusion that TA was never held at gunpoint in the shower. On that I think he is very much mistaken. I don't think there's any reason to accept sex that day just because JM thinks that, or even simply because of the photos.
But....I remain pesky about a reasonable why, and an alternative timeframe that fits.