But my initial reaction was that I would find him cute, but immature. Sort of like meeting a much younger brother of his. Reminds me of him, but he's just not the same.
Your reply is very interesting, but in the case of this story, I still think the two mismatched, mixed up couples' feelings of confusion etc wouldn't be the only ones they'd have toward each other, nor would the disparity between them cancel out the basic romantic attraction that would arise from simply being in proximity with each other, especially with the threat of permanent separation looming.
Or maybe my experience is just different. My partner has always been a very together sort of guy, and I don't recall him ever being that immature. The only difference I would say is that he was more unfocussed back then; he has more definite ideas now. (I think that applies to PC too; Lois was always the more immature one, and he always seemed, in this show, very together).
If I was able to interact with my hubbie as he was 15 years ago, I don't think I would be amused or affectionate; I would be more intrigued and interested. Plus we both suffer a bit from a middle-aged spread, and he was really hot back then (not that he isn't still now, but a younger version -- YUM) and so I would love to get my hands on him, likely ignoring any ethical quandries or future consequences.
But naturally that doesn't apply to L & C because Clark's physique is always perfect (of course!) and maybe as a couple they're above such physicality since their future is at stake. Yet a Clark suffering from unrequited love, with all that sweetness and hopeless longing (remember the end of the invisible man episode?) -- he is my favourite Clark. I just can't see a Lois who has been deeply in love with him for years (since the end of season one probably), and who has all that deep knowledge of who he really is (which still applies whether he is PC or FC) having sisterly feelings for such an amazing guy, or even being able to keep them separate in her mind all that easily. Just my two cents.
You should know, though, that even if you decide to keep FL & FC and PL & PC forever separated and living out their lives in platonic friendship, I'll still keep reading this story, no matter what (and avoiding any work that needs doing because of it).
