It seems like for most people this is always hardest the first time, makes little difference where/when it happens. Chances are very good that it will get easier. Don't you think you'd be just as jealous if he were taking time away from you to be with her, maybe even more jealous? At least this way it doesn't impact the amount of time that you and he can spend together.
I do feel compelled to say that feelings happen when you least expect them. You both may want to consider working on loosening your restrictions on developing actual relationships with other people now -- him too, not just you -- before someone ends up feeling betrayed. After all, how many bfs/gfs start as casual flings? Feelings happen.
Great resources on dealing with jealousy here: www.morethantwo.com
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.