I'm torn here. I agree yes your DD is an adult. However it is still your house and if that makes you uncomfortable then there is nothing wrong with that.
For the record I wasn't allowed to share a bed with a b/f at my parents until I was 25 (yep !!!) and even then my Dad was dead against it.
Before then it was sleeping on sofa. Their house , their rules.
My sister however who is a lot younger than me got it far easier!!!
As it happens now, me and DP stay all the time at my parents as they live 2 hours away and we never have sex in their house. It just feels disrespectful and uncomfortable. And we love sex! But we both feel the same about this. DP even wears his boxers in bed "in case there is a fire" I kid you not. I'm 38 btw!!
My advice is maybe say yes, only on sofa but be prepared for her to say no.
Maybe wait until they are further established then say yes.
Or perhaps explain ok yes, but that you feel uncomfortable with any sexual activity under your roof and you'd rather they refrained from it whilst staying at yours.
You are still entitled to your opinion and there is nothing wrong with that. Yes she may ignore you, but like me, she may also feel a bit uncomfortable.
She is an adult so perhaps have an adult conversation with her and if she gets funny then remind her she's now an adult and if wants to be treated like one then sometimes you have difficult discussions