My boyfriend does some things that make me uncomfortable, like dating much younger women and doing drugs with them

I'm a 21yo girl and I started dating a 27yo guy a few months ago. At first he seemed a little weirded out by our age gap, but we had so much in common and meshed so well that we decided it didn't matter. Fast forward two or three months, and he's dating an 18yo girl! What? I'm weirded out by it. I'm still unsure if polyamory is right for me. I like being in an open relationship, but I think I want a primary partner and the freedom to have hook ups & casual flings as opposed to multiple serious relationships. So that's definitely a part of it.
Is it any of my business who he's dating? I'm curious about polyamory (like maybe I'll meet another guy I want to date and decide I do want to have multiple relationships after all). But ugh every time I think about him with such a young girl I feel uncomfortable. He's also big into party drugs and it makes me uncomfortable thinking about a 27yo giving an 18yo drugs. But at the same time, I don't know the girl and maybe I'm judging him too much. I just don't know how to navigate this.

It is entirely reasonable to be uncomfortable about the fact that a 27 year old man is in a sexual-romantic relationship with an 18 year old woman who he is doing drugs with. Saying that he’s “weirded out” by the age gap but then deciding it doesn’t matter is a very common tactic that older people use to get a younger person’s guard down, so don’t give him too much credit for that.

If you’re already unsure about whether the relationship is right for you, and you’re uncomfortable with your partner’s behavior, and you’ve only been together a handful of months, it’s probably best to gracefully move on from the relationship.