bf wants a threesome but can't even last long enough to satisfy me😂😂

so my bf and i have a kid together. he left us when our boy was just 4 mos old. we were broken up for 6 mos. he just recently came back to us after graciously granted him another chance. sex was freat the first month, now not so much. he barely ever makes me cum and its always me who initiates sex. so it sounded so ridiculous when he told me he wants a 3some😂😂 like boy you can barely last 5 mins with one girl who tf are you kidding😂😂idk it was just so annoying like no way in hell im agreeing to it when i caught him messaging girls behind my back. i should be his only priority not thinking about other girls😤😤