I can identify with the 'pleaser' mindset. I became someone's secondary mainly for that reason, once upon a time. The wife, also a pleaser, tried to go along with it but I don't believe she was ever really okay with it. The lesson being that trying to please some one else is a terrible reason to try poly and usually ends with people getting hurt. If at some point, you think about poly and feel good about it, great! Go ahead and try it. But it's really okay to look at it and say, hey, this isn't for me. Even if that means losing the relationship.
Here's the thing. Even if you can control who, what, where, when, there are still so many elements out of your control. Like feelings. If being in control is the only way that you feel safe to step into this, it's a fantasy. That's not how it will end up. Granted, boundaries are important in polyamory but there's a difference between good boundaries and running your relationships like a lab experiment with controlled variables.
Don't be in a rush to please your boyfriend with this one. There's no shame in wanting exclusivity. Nor in being straight. Listen to what you really need beneath all those pleasing instincts. You might end up between a rock and a hard place but it's usually better to pick the one that let's you stay true to yourself.