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"I also feel like he should go on dates with other women, and have casual sex too, but I think he's too insecure with himself and our relationship for something like this."
I take it you're speaking of your boyfriend here? (rather than your other friend.)
Do you and your boyfriend get a chance to talk much? How does he feel about your other friend?
You have to talk and be honest with one another. It sounds like you want something like a FWB relationship with your other friend. It sounds like you and your boyfriend are both afraid of losing each other. Are you both committed to staying together? Perhaps a commitment ceremony would help?
We'll help in any way we can.