In many ways, I can empathise with your boyfriend.
The being able to deal with lust but not love was a situation I was in at the start of my polyship.
I would suggest giving him time. Don't let it go, still mention it but don't push too hard. Let him know this is something that you feel you need and would make you happy but don't expect him to accept it right away.
Also, I would suggest that being sexual with others in the mean time could possibly make things more difficult for the both of you. If you have a sexual relationship with someone and develop feelings for them you will be hurting (as your boyfriend is not ready for that yet) and your boyfriend may feel rushed and have a more difficult time if you develop strong feelings before he has emotionally equipped himself to deal with them.
I think it's usually easier to get someone used to Poly theory before you dive into Poly practice.
However, you may develop feelings for someone anyway but at this stage, if you did and you acted upon it, it would be considered cheating by your partner and would make it more difficult for him to accept.
Of course there is also the possibility that he'll never accept it but I think it's worth giving him time to mull it over first. Some people need to really think these things over.
I took my time at first. I also needed to have several tantrums before I felt okay with it, but now, a year and half into the relationship and my boyfriend says he thinks I've taken to poly like a fish to water - in fact I'm more active in the dating scene than he is.