Bit of a different take from many of the other comments.

Relationships don't have to be the way they are traditionally. You don't have to be monogamous. You can be in two relationships, one of which is romantic, one of which is a strong friendship with sexual aspects. It'd be under the polyamory umbrella. There's plenty of potential partners out there who are ok with or would even want this kind of a constellation.

The very important caveat though: even more so than in any other kind of close relationship, this requires a LOT of communication. You need to clarify with your best friend what kind of a relationship it is that you have. You don't have to label it, but you have to figure out together what you want from each other and what your boundaries are. The latter includes what kind of relationships you're ok with the other one having with someone else. Then, when you date someone else, you have to have the same conversations and be open about your other relationship(s). I'd be upfront about the latter, the former can happen over time.

If none of this sounds like something you'd want, that's of course perfectly valid. The point stands though that you need to clarify with each other what you want from your relationship and what your boundaries and needs are. This might mean having to change your relationship dynamic.

Some context: my best friend and I are super close and find each other sexually attractive, but aren't romantically interested in each other. We've talked about that and keep checking in. We've done some second base things and cuddled. I now have a partner that's decidedly monogamous. Now, I might still lightly cuddle with my best friend, but we wouldn't kiss anymore. Those are all boundaries that had to be talked through.