Maybe talk to DS1 about it. Instead of talking to a teen, talk to another adult iykwim. If he & his gf can have their space at her house, you aren't exactly taking that away from them.
Maybe sit them both down? Say you need to get their point of view. Explain the whole situation & that you hate the idea of taking their privacy away, but that you need to think about everyone. If you make them part of the decision, rather than just tell them thats the way it will be, it might be alot easier, with alot less to deal with from DS1 iykwim.

That sounds like a great idea. so whilst you may well have already made up your own mind and will do it regardless, he can at least feel like an adult wth some input, and it will also make him feel like he has saved some face with his girlfriend, rather than looking like a little kid who lets his mum make decisions for him

Good luck - not easy, but necessary for everyone.