I'm not so sure that the state was convinced that there was sex that day, but rather that it was easier to accept that there appeared to be, with little to be gained from closer analysis - it would be impossible to definitively say one way or another. And of course it didn't hurt to go with the flow, and be able to portray Arias as a true black widow, killing her mate after sex. But perhaps you feel the same way, as you say they "conceded".
And perhaps I'm naive, but I really can't imagine Travis falling for any scheme or sexual temptation at that stage. I know he's a guy but I think that even for the most lusty male, there's a line that's drawn. Plus, Travis had been lured in many times before, and her stalking, etc., behaviors had if anything worsened. I think he'd learnt his lesson, and then some - that he would have been utterly disgusted with himself to be drawn in by someone he'd come to thoroughly despise. But most of all, because Arias knew that's how he felt. That's why she was there, to kill him.
Apologies if I'm repeating myself - will cease and desist!