Can Someone Explain This To Me?

So, I heard someone this morning mention an arguement I find ridiculous. They said if we let gay people marry, where does it stop? People will start marrying their dogs and horses, etc. I've heard this arguement 1000 times and none of them has it made sense. If 2 people who love each other get married, how does that lead to people marrying animals? The fact that the two people happen to be of the same sex does not change the fact that they are 2 human beings who are in love and want to express it. If two people of the same sex getting married leads to people marrying their dogs, how does 2 people of the opposite sexes not lead to this same thing? Can someone please explain the rationale behind this arguement to me? I have never understood it and every time I hear it, it makes me think the person saying it is a complete idiot. So, a good explanation might help me get away from that thinking. Thanks!