I want to love my best friend even though I know it will hurt him
So, I have a boyfriend, but I want to sleep with my best friend. It's nothing like "I'm not attracted to my bf" Or anything, I love him very much... Both my bf and my best friend started out loving me, but I only wanted them as friends. My best friend understood me not feeling the same, and only wanting to be besties. It hurt, but he understood and dropped it completely... Or so I thought. As far as I know, he still loves me completely-- But with my Bf, it was different. He wouldn't stop being a gentleman and trying to win over my heart... After about six months, he suceeded. This broke My bestie obviously, and he was hurt for a long time... About two months ago or so, things got a lot better, he stopped stalking me on the app where I talk with my bf, and he didnt get so angry or depressed anymore. He's the amazing best friend I've always known ^^ .... But now, I find myself daydreaming about having passionate s** with him, and being a couple, and having fun together, and flirting.... I know I wouldn't want to marry him, because (As cliche as it is) He's not my type. He gets jealous and sometimes depressed, and he's often distrusting because of a lot that happened to him in the past. I could never marry him... But i'm pining away at the thought of having his love, for a while... I know if I acted on this, it would ruin everything though. Because once it's over, things couldn't return to how they are now, with him being my bestie, we'd drift apart.... He's the only true best friend I've ever had, I don't want to lose that... Any advice?
TL;DR: I'm commited to my Bf, but want to have a temporary love/fwb relationship with my Bestie, despite knowing it will ruin everything and we could never go back once that line is crossed. Anyone have advice?