A serious relationship implies a certain level of mutual dependence. You are a woman, you are used to constant dependence on people and things, you don't see it as a weakness because it is a big part of who you are. And trust me, your dependence on people or things (your weaknesses) have been, and will continue to, be leveraged by people to get what they want out of you, whether you know that's what they are doing or not. NOW when it comes to the OP I stand by my comment. This late in the cycle it is unlikely there are many seats available, for the sake of argument let's say there is just one, but out of the interviews emerge three qualified applicants, two of which are our couple. The committee now knows the two are attached. Who are they going to choose? The answer, my friend, is bachelor #3. If you think the adcom would make their decision without considering the emotional toll it would take on the couple if they chose one over the other, you are naive. If they separate the couple they knowingly admit a student who will matriculate with extra baggage which could very easily affect their academic progress. This wouldn't be a responsible decision. Moral of the story: Don't own up to being together and increase your chances of being accepted. If it were earlier in the cycle I would argue that it would be less likely to negatively affect ones chances, in fact, it may not affect them at all. There is no scenario, however, where making this known to adcoms would make your chances better.
I hope this is another typical ewu comment, I wouldn't want to disappoint you sweetheart.