Let's try to analyze this.
I agree about her lifestyle choice being openly immoral, at least by the standards of Red State Morality - the Judeo-Christian tradition. But I catch that the real key is her openness about it. If she was having sex with her boyfriend before this decision, then isn't it really the public nature of this that is so galling? The personal part is, she knows how you feel, she still throws it in your face, and she still expects you to keep up your generous support. What a slam, in my opinion.
This young lady and you are very far apart, in your relationship, and it didn't begin just now with her unfortunate, recent behavior.
About the tuition...I think you should try to separate the decision from a narrow focus on the living with her boyfriend situation. Look at the widest possible scope here. What else does she do, or not do, that shows you if she is maturing in her adult responsibilities? She is young, but since she is depending on others for support, and since she is making very adult-level decisions, she needs to be held to certain expectations of increasing maturity. Can she work and contribute to her education, even a little? Can you then help her out some, but not foot the entire bill? Is her boyfriend a bum, a loser who will mooch off of her or her mother or you? If so, that is a very serious problem in itself.
Don't try to control her with purse strings, even though she is wrong, has hurt you, and you are angry. Instead, let her know you are concerned for her having a good life, whether in a good relationship with a man or in her education/career etc. Make it clear she is going against your moral values, but you understand they are yours not necessarily hers(sadly). Say you want her to make good decisions and mature in her ability to show responsibility in life. Gently remind her that she is not yet an independent actor in life, as she still depends on others to support her, whether financially or emotionally, therefore she should try to maintain happy relationships with those she expects to be "in her corner".
And tell her that you will try, with all your heart, to maintain a happy relationship with her, because she is your flesh and blood, and you will always love her.
If you feel unable to tell her this face to face, why not tell her in a hand-written letter?