Just to throw in my 2 cents, I think Alex and Isabel's relationship simply has a very pure feeling to it. They're love doesn't seem based on physical attraction at all (not to say that they're not attacted to each other). Alex isn't the guy to fall for a girl just because she's pretty, he always seemed to see the real Isabel, even before he really knew her. And Isabel fell for Alex because of his, well, because of his pureness I think, even if that sounds stupid. He was loyal, loving, kind, sweet...all the things girls want in a guy (if she's smart, that is). He would always be something stable in her world, no matter how messed up the rest of her life might become, another thing Isabel craved. Besides, he accepted her just the way she was. He never tried to change her, even when she wasn't exactly treating him all that well. I think in her eyes, he was simply perfect.
I guess Stargazer stories just focus on the non-sexual aspect of the relationship because it's what stands out the most. With Michael and Maria for example, there was much more physical stuff in the foreground, same goes for Max and Liz, which is probably why there are more NC-17 fics for those couples. Alex and Isabel just kind of go beyond that.
