I don't think there's necessarily anything wrong with what you want. It simply is what you want. As long as you can find a girl who also wants the exact same thing, I don't see an ethical issue.
However, finding a girl like this is probably going to be your first challenge.
Your second challenge is going to be keeping the relationship that way if you can find it.
The third challenge is going to be to remember that she won't be an extension of your relationship with your boyfriend. Your relationship with her will be its own entity, with its own life. The same for her and your BF. All sorts of things could happen.
So, trying to find the girl: You could try joining swinging and open relationship communities and groups. Searching for events in your area. There are a number of swinging dating websites, where you might be able to find someone open to this particular dynamic. You could also try OKCupid, which I've found to be one of the more poly-friendly options. (Though, many people there identify as polyamorous rather than polysexual or otherwise).
I would suggest, firstly, that you seriously think about your expectations, hard limits, guidelines, etc. It can help to actually write these down, make sure you both agree on them and review them regularly. When you meet a new girl, you'll have to be very, very clear about what they are and make sure that she really does agree.
Genuine questions to ask yourselves:
- Is falling in love with the girl ok?
- If no, what steps will you take to prevent it? What happens if someone does fall in love?
- What happens if you meet a girl you *really* like, but she's not into BF?
- What if BF isn't attracted to her?
- What happens if the girl is open to both of you at first, but stops wanting to have BF involved?
- What if BF and the girl become closer than you and the girl? Or if they want to have sex/relationship alone too?
- What if BF eventually decides that he also wants to date others?
- What to do if someone's feelings change?
Practical guideline ideas:
- How often can dates happen with the girl?
- How long can the dates be?
- How much will BF want you to share? Where is the line for the privacy of the new girl?
- How much texting/relationship maintenance is acceptable?
- What are your safe sex agreements?
- Are overnights ok when it's just you and her?
- Are weekends/trips away (you and her) ok?
- What if it starts well, but she and BF fall out?
I would say that the biggest word of caution I can give you in this situation is that relationships tend to have a mind of their own, and muddy guidelines / poor communication can have catastrophic results. Hearts can be broken and relationships can be ruined. It's important to remember that relationships are seldom static and that various emotions/limits/wants/needs can develop. It's also important to make sure that you and BF trust each other enough to be open if you start to feel something or have done something that your guidelines have specifically disallowed.