I prefer it as a permanent love triangle, all the more poignant because he never had Aerith, and now she's dead, and Elena will always wonder if she's just a substitute.
I don't think Tseng really knew Aerith, I think he had this idealised idea of her. I dunno maybe watching over her made him feel better about all the killings and kidnapping and other horrible stuff he did. Anyway, I don't think thats a good foundation to build a relationship on. Also slapping just isn't on....unless it is on the buttocks and is semi-consensual
Anyways he DID ask Elena out before Sephiroth ruined everything.