The assumptions that I will be making over here are that the people in question here is matured enough to take their careers seriously and also that they are looking at a long(-ish?) term commitment towards their respective amours.
My theory regarding this comes from my experience the last time that I had a girlfriend, back in engineering college during the second year. Before I had a girlfriend, my grades were pretty decent, but that was because college had just begun and I was pretty serious about my studies. In the second term, I found someone special, and my grades just zoomed up like anything! It was a long distance relationship, and predictably enough, we broke up by the end of the 2nd year of college. I went into mourning, and my grades went through the floor, and probably emerged somewhere in Colorado. I didn't really check where they went for about a year, and then by my 4th year of college, I had gained some control of my life and managed to put up a decent show by the end of engineering college.
This said, my theory says that having a girlfriend gave me that focus in life, knowing that I was loved by a beautiful beautiful person and that I also loved her back. I guess this would be generalizing too much, but I think that as long as I did not have a girlfriend, I focussed on trying to find one, trying my luck out with different girls, spending time networking online, offline, through friends and so on. Post the breakup, I spent time moping around, getting philosophical, trying to figure out what went wrong, where I could have behaved differently, and doing everything except studying. Sure enough, my grades plummeted.
So my advice to budding lovebirds would be to get a girlfriend/boyfriend and stick with them for a long term, keep a bigger goal in your life so that if you just have your amours as your sole goal in your lives, your world will crumble around you, like mine did 5 years ago, but if you really want to do something with your lives, then a small hiccup in your love life shouldn't affect you as much. It should just inspire you to work harder towards your ultimate BIG goal, and then maybe find another somebody special along the way.