Hi Scarlett. IMO the sexting is tied into motive, and is related to his searches on the 'child-free' lifestyle--he really did not want to be a 'daddy' tied down with a wife and toddler as evidenced by his profiles on hook-up apps and liaisons with under-age girls. Even his wife said she knew he "wasn't getting what he needed at home," whatever that means. Plus, to me it speaks to a degree of depraved indifference--if you know your young son is roasting to death in the back of your car (and I believe he knew very well) then what does it say about his state of mind that after the mid-day trip to buy lightbulbs and take them out to his car he was sending pics of his erect penis to girls?
So, in my mind his sexting actions are not separate from the crime of deliberately letting his son die in a hot car, they are intimately (pun intended) tied up in the whole mindset that led up to the crime and carried him through the commission of it. Of course, many people cheat and don't go on to kill their children, and some others remain faithful but do go on to kill their children and there are many variations on both themes, but in this case I think the acts go, er, hand in hand.
YMMV!