Not the reason you think. Actually, the physical act of "making love" would probably result in Lois's rather quick demise. Remember that at the moment of climax, Superman would temporarily lose control of himself. Superhuman "thrusting behaviour" would be more than a mortal female could survive. This is why Superman is a virgin. parsy who actually thinks about stuff like this.
Uh, first of all, Superman is MARRIED to Lois Lane these days. Has been since, I think, 1992.
I stopped following Superman before 1992. Straightened up did he? That's good. Any kids yet?