I agree with everything except the last part. I don't think Touma was opportunistic. Mainly because he wasn't the one to push for sex. Sure he was the one who pretended to be Rikka first, but that was only to get Aki to eat. To not have Aki become so consumed by his sadness that he loses all will to live.
Sure, he could have said no to the sex but we have to remember at that point his original feelings for Aki had probably resurfaced. It would be hard to say no to the man you love who is in such a fragile state of mind that his current mentality might break if set out of balance as it did at the end. (When he killed himself)
But apart from that I totally agree.
Idk, I actually thought that the ending was for the best, he deserves someone better than Aki that truly loves him, Touma just simply was not loved by Aki who loved Rikka, have to keep remembering that even a hetero pairing in a yaoi manga can still be real love lol, seeing it from that point kinda makes Touma a manipulative opportunist