The issue is not whether or not your daughter is having sex - that is a given in this instance - but whether or not you are happy for her and her boyfriend to share her bed in your - once again that is YOUR - house.
If you are not comfortable with the situation, you are perfectly entitled to say so, and your daughter should respect your wishes. It's not a matter of telling her what she can or can't do, it's a matter of being comfortable in your own home.
The fact that you ask the question suggests you have some misgivings. If you are comfortable fine, but don't do it just because his parents do. Have a good think about it, and go with what works for you.