my two cents...
Your boyfriend finds it easier to hear about you being with another woman, than being with another man.
your boyfriend needs time to adjust to you having a relationship with another man. give him that time.
you are not being upfront and honest about doing things with other men - prior to doing them.
this would help with point 2 i made above.
what you have at the moment seems to be an open relationship with an "i'll tell you when i get around to it' policy... not really a fair way to do things IMO.
it sounds like you guys really need to sit down and hash out some more detailed boundaries, and really find out where each of you stand on different issues.
*boundaries may be the wrong word - you guys seem okay with the openness - it may simply be that communication is lacking, and understanding of your boyfriends feelings. I would also question why you only feel the need to be with someone else when you and your boyfriend are physically separated.
This to me is not truly polyamory - but an open relationship... you didn't mention being in love with any of the people you slept with and also didn't mention love between your boyfriend and your best friend...
apologies: I just reread the last paragraph. you do feel you love the last person you mentioned and wish to continue your relationship. this is where it gets messy
read my threads - one is below in my signature, it can get really messy. most important is to talk openly and honestly to your partner... and to respect his feelings, really respect them... put them first, but also do not deny how you feel and work out a compromise with a date to renegotiate how things are going.