Ditto.
Next, you WANT to keep an OPEN communication and relationship with your daughter. VERY important.
BUT... respect her. She has grown up now.
This is a "milestone" and a point of growth for YOU too....
it is growing pains, for the Parent.
Just make sure your daughter is WISE, and takes care of her birth control, talk with her about it, and make sure she is AWARE of STD's.
AND, if she is not already, make sure she has an OB/GYN that she goes to every year for check ups. Now that she is sexually active, it is even more important that she do this.
Make sure she is on the ball with these things.
AND, ALWAYS be there for her for any disappointments and any heartaches... she will need her Mom (& Dad) to be there FOR her.
At this age, relationships do not always last "forever." But she will need to learn that on her own. AND she WILL have other relationships too.
As for the guy... you aren't his parent... so be careful of boundaries... but, for me, My Dad would sort of instill what behavior he expects while in OUR home. But don't be too heavy-handed in that... or you will undermine her relationship.
Next, despite your feelings on this matter, she is 20 years old. She is an adult. If you keep telling her not to have sex and "lecture" her on abstinence... she may very well get tired of it and then just STOP telling you anything. THAT is worse. AND she may just lose any trust in you, and then may never come to you for confidences.
Whatever you do, talk WITH her as an adult. Don't treat her like a child. She will resent that and then won't open up to you.
Your job now, is to keep in her trusting circle, be her Mom, Be her Dad, and be there for her.
For me, my Dad was a BIG influence on me and when I dated/had relationships. He never judged me. I even went through a phase where I dated a "punk rocker" in a band, who would pick me up on his motorcycle. My Dad never flinched, he even liked the guy... the guy was very smart/college educated/and a gentleman after all. BUT, my Dad was always there for me, I could tell him anything. THIS is very important. He never talked down to me or judged me... but treated me as the "smart/wise" daughter he knew me to be and trusted that EVERYTHING that he and my Mom taught me... was guiding me.
My Mom on the other hand, was more critical... and so I didn't tell her as much as I could tell my Dad.
So keep that in mind.
All the best,
Susan