This was before Watchmen, btw. Micro-Lad is left-over from a weird political storyline where they replace Shrinking Violet (the angry woman beating on him) with a shape-shifting spy.
This was years before Johnny and Lyja. This had the same denoument as J. and L., too, except the shape-shifter turned to good and married...Colossal Lad.
Because S.V. is a lesbian.
Seriously. She's a lesbian, and she loves Light Lass. It was the first lesbian pairing in mainstream comics. They were together at this time, though the creators would not make it "official" for another four or five years.