he is also happy because she likes him, being a fat guy not many people like you, and he probably feels like he is dependable, if they don't stray too far with the story you will understand more about this later on, Hirano is a really cool character, definitely not a pedophile, at least not the impression I had from reading the manga.
thats how I felt when someone mentioned my Saya hooking up with him. . .
and no. . .this is debating like friends not discussing like enemies

and Saeko is on my good side so I wouldnt really like if she hooks up with him (it was just to tease Eseuldor which IIRC is Saeko fan)
too late for excuses now mate, I'm a big fan of Saeko's as well, at the very least you could have hook him up with Rei, teasing like that with Saeko is bad for your karma

also, Rei redeems herself along the way, so I will stop hating her unless the director will go too far with deviating from the story, don't understand why he does it, so stupid I can't even describe it.
Edit:
I have spent a few minutes and been constantly re-watching the preview over and over again.
I noticed something and we are a bit mistaken, Saeko doesn't get the sword, the one in that small shrine is a part of the Buhdda Statue and in the later screens, she uses her wooden sword still. If them being separated at this stage is only a short time, I don't mind them showing the true Saeko now, especially if what I think will happen by the end of the second half of the next episode will happen it will be quite funny. Still, Deviating from original Manga content is not a good idea, and they done so several times already, it didn't affect much the main story, but they start doing it more visible, and more annoying.