What you are talking about is a fiancee visa and of course there is no obligation to actually marry your "fiancee". But, from what I have heard, fiancee visas are difficult to get. You have to provide a lot of supporting evidence.
In my opinion the entire visa process is highly subjective and a big pain. In my case I was already married when I applied for my wife's visa the first time. We gathered all the documentation we could think of to use as supporting evidence for her situation and our marriage. Got to the Embassy for the interview and the woman working there did not look at a single document. Simply said no. The reason given was that my wife had no sufficient reason to return to the DR. I have heard that this is the blanket excuse that they always give. They say that to get one you need to show evidence of good financial standing in some way. Not rich, but definitely enough money coming in to pay for the trip and your return on your own. So it wouldn't matter if I have the money and I am her husband. But nonetheless they didn't even look at bank statements. Another thing that they supposedly like is if you have minor children or a very good job. My wife has neither of those.
Funny thing is we reapplied again this year and absolutely nothing has changed. My wife's financial situation, children, job and everything are identical. In fact if you compare the applications and supporting documents the only difference is the dates on them. Guess what - she gets a 10 year unlimited entry visa. WTF. I mean I'm not complaining that she got it, but why no the first time and yes this time with nothing having changed at all.
But keep in mind the blanket excuse I told you that they like to use. They really want to feel good that the person applying for the visa isn't going to show up in the States on a visa and then stay there forever illegally. They want to see a reason for her to return to the DR. The fact that my wife and I have been living here for years myself and she was my wife did not matter one bit the first time. So don't think the fact that you are living here and she is your fiancee is going to make any bit of difference. You are not even married yet. So if she doesn't have her own good reasons for returning here it will be very hard to get the visa. Minor children and a very good job are the top two reasons. Plus she needs to show she is financially stable on top of that.
Maybe you are trying the angle of bringing here in as your fiancee because you think it will be easy to get that one granted but I have heard they are not so easy. It takes a lot of time and costs more money than the regular tourist visa that I got my wife. But if you want to try this route because you don't think the tourist visa will work then you need to be ready to show that you are really a couple that is really intending to get married. From what I hear they will want lots of evidence that this is a serious relationship. Even on the visa website they say that they may want to have letters from family, photos of you with her family, etc. Its not a slam dunk by any means. And it would suck to go through all the aggravation of one of those and then not get it. At least the tourist visa is a fairly easy process and only costs $160.