To avoid broken hearts, you have to choose carefully who you romance. Nell and Evelyn are the only characters who you can't have sex with at all without romancing them. Getting a "friends with benefits" relationship with Primrose is a bit tricky, but it can be done. Having sex with as many ladies as possible without leaving any of them unhappy means picking either Nell or Evelyn as your love interest, skipping the other completely, going FWB with everyone else, and turning down the offer of kingship. If you prefer to marry someone else, you have to skip both Nell and Evelyn.
Because you need to max out your charisma to win the game, you do have to pursue the relationship paths with Nell, Primrose and Callie to get their charisma point rewards, but stop short of saying yes to becoming a couple (with Primrose you get the award regardless of whether she offers romance or an FWB arrangement). You get Evelyn's charisma point when you remove her incest/infidelity inhibition, and I think you only get Samarra's if you take advantage of her after defeating her in combat.
Tip: Choosing to "pursue a relationship with the Fairy Queen" at the waterfall early on does not count as a romance, despite the wording, so you can get the "true love" achievement with anyone you like while also enabling Thorn's survival in the epilogue.