You're happy she's in love but you don't want her to sleep with him in his place? Is that the dividing line? How do you know where she will sleep? Where did you get the estimate of 3-4 nights a week? From her? Before she met this boyfriend, assuming she was in college for 2 years, how do you know where and with whom she's been sleeping? Maybe this is a terrible idea, this boyfriend. But that's what young people do - they date stupid people and they regret it, or maybe they hit it right the first time. Who knows.
I assumed, when my kid went to college, that I had given him all the structure and guidance I could up to that point. After that, my job was done. What good would it have done for me to prohibit or limit his social relationships?
I think 1 of 2 things will happen.
1) This guy will relocate and will find he has nothing in common with a 20 year old college student and her 20 year old friends and their campus life.
2) You will drive her into his arms by forbidding it. She won't have any money so she will drop out of college and align herself with an older man because she feels her parents have abandoned her. Not that you have - but she will see it that way. And she won't finish school.
Or you can choose #3: You raised her, you trust her, you value her brain and her ability to handle a college course load. You taught her to make decisions and so now you have to let her. She trusted you enough to tell you about her relationship, and now she feels judged. I think if you say "you're a good girl" she is going to hear you say, "you're a good girl, but..."
I think you agree to pay for your original agreement and perhaps it's based on her passing courses and even contributing through a part-time job - whatever the deal was, if it didn't include "so long as you never have a boyfriend you sleep with," she hasn't violated anything.
It doesn't change your financial costs if she stays in her place, his place, a friend's place, or your place on vacation. You have to keep the relationship open. You can say you don't want to talk about her sex life (and she probably doesn't either), but I'd stay out of it. She's 20, after all.