He proposed to her in Superman #51, which was published in 1991. I remember this because this was at the start of the triangle system that lasted until 2001. Right after he proposes, he reveals his identity by pulling his shirt open to reveal the S-shield/symbol, a moment I was already aware of thanks to research but still felt it in the writing, it was so good. Plus I get caught up into moments like that anyway, I'm such a hopeless romantic. >_<
Anyway, it was many years until DC finally got them married in Superman: Wedding Album, which was published in 1997. The reason for the gap is that DC wanted the wedding from the Lois and Clark: New Adventures of Superman TV show to coincide with the comics version, mostly to boost sales and gain new readers.