I've only ever had sex for the bond that you create with someone. It's like a trust that nobody else can have.
I think in order for you to become comfortable with your boyfriend fucking and loving others, you need to address the sentiment of possessiveness behind this statement. If you are already seeing the bond created through sex as something exclusively yours to have, you will struggle. For many people, physical intimacy does lead to greater emotional closeness. You say your boyfriend meets your needs; the issue is that you are not sure you meet his. If asking him, point blank to his face, doesn't satisfy you, then you need to delve deeper into why.
I don't think it's irrational to worry because it does sound like there is a genuine disconnect between the pair of you. I hate to say it but it doesn't follow (to my mind at least ) that just because you don't particularly enjoy sex, that him finding super awesome satisfying sex elsewhere wouldn't be a threat to your position in his life. It might be. So, what would convince you otherwise? Poly might be a solution, because if you set him free in that way, you might find that he keeps coming back to you, so to speak. But if him having a satisfying sexual/romantic/emotional relationship with another is going to mean you constantly feel inadequate or less than, then it's not viable. In all honesty, polyamory doesn't tend to work unless both partners want it for their own reasons - not just for one's partner's own sake, but for oneself. What could having an open relationship bring to YOUR life, other than potential satisfaction for him? When you have the answer to that question, you will have a motivation strong enough to keep you working at your insecurities.