I understand very well that you're thinking about this now, because it's not just about how this specific person will react, but more about what you want and what you need. Some things come up in your post already:
- you need the freedom to appreciate the beauty of others and flirt and cuddle with friends, but you want to be able to have more
- you don't think the relationship with you lover is enough to satisfy all your relationship needs, but you enjoy it nonetheless and you want to continue the relationship
- you want/need something more serious? What does this entail? Commitment, primary-type-relationships, more time together? Do you think you could get your needs met in a few less-serious relationships, or is "serious" what you're after?
I don't think it is impossible for you to find a man, who is either poly or willing to try poly for you, but obviously it will limit some of the options. You have some worries about that, though. If the person you're interested in would need more support than you are willing/able to give then I guess you wouldn't be compatible. I would think that if you find somebody you truly want to be with, you'd be willing to make yourself available to support him. What do you think? Then again, if you'd have a hard time if/when he finds another love, you do have a whole forum to turn to for help, and I'm sure you can work with that.