I FOUND IT!!!
Sorry for the caps lock, but I've been looking for this movie since I saw it when I was a little girl. I could never remember the title, only the general plot and that it ended with the man and woman walking off hand in hand and the words "The Beginning." Over the years I've tracked down all the movies from my childhood that I forgot the name of, and this was the last one! I finally figured it out by just Googling "50s scifi movies" and going down the Wikipedia article "List of science fiction films of the 1950s" and clicking on whatever title sounded fitting. And I found it!
Oh my God I'm just so excited. I've been searching for this for years! I remember how much fun me and my family had making fun of this. I also vividly recall that we were having lasagna for dinner, and the... something (radiation burn? Mutation?) on the bad guy's face looked disturbingly like the Ricotta cheese on the lasagna. I didn't eat it.
"He's already attracted to her. Time and monotony will do the rest."