what can someone who is so open to romantic relationships do to close off against the same? I have the right number of people in my world but I continue to have these silly little sexual urges
I think that it might be premature to worry about this at this stage. In my (arguably somewhat limited) experience, it's not inevitable that you will start to develop romantic feelings with someone you connect with sexually and on a friendship level. Even if you are the type (as I am) who does get emotionally closer to others via sex, because you probably won't be looking for someone with the same qualities as what you typically look for in a more romantic/committed partnership, it can have a very different feel about it. Hard to explain until you've been there, but I don't feel like I have to consciously close myself off to a more serious relationship - the friends-with-benefits type relationships that I have explored to date sort of just find their own level.
I'm currently dating a guy that I connect very well sexually and intellectually with; we are definitely friends as well as lovers. But he has a number of personality traits (as I'm sure I do with him!) that mean we would be ill-suited as anything more serious. We make time for each other when we *have* time, and so far it seems very well balanced in terms of not leaving either of us over-stretched with regards to our other partners. In short, if you don't want another serious long-term partner, don't look for someone that ticks all those boxes for you. Be up front about what you want (lots of hot sex, not so much on the commitment/romance aspects) and what you can offer (lots of hot sex, not so much on the commitment/romance aspects) and relax. You sound like you know your own limits and how much space you have in your life for other connections, and I think that puts you in a good frame for moving forwards.
Also, your sexual urges are not silly. They are important to you, and you shouldn't feel weird or awkward about wanting to feel fulfilled in this way.