An interesting idea to pose. What if you don't get the ATAR you want and it's perhaps because you were getting too distracted with your girlfriend/boyfriend? How would you then feel if the relationship didn't last? Just want to hear people's thoughts, that's all. 
Never regret your decisions because at one point, they were what you truly believed you wanted.
Don't let year 12 stunt a budding relationship, IMO. It may be a tad harder to manage your time (especially if you are a new couple), but I have, and many others, have achieved more than they expected in VCE with a significant other.
I asked out a girl two weeks before my English exam cause you know, you only live once and all ;P
P.S. We broke up well before the English exam.
Was it a stupid decision? Yes. It was pretty stupid.
Did it hurt my English score? Kind of.
Do I regret it? Not at all, cause at the time, it was what I wanted.
So don't worry so much guys, in the end things are just what you make of it, look onto things with an optimistic eye and you will be rewarded for it.