I have a question about a boy i fancey?

i am 13 and not really wanting to now but the boy i fancey asked my bm if i would sleep wiv imm etc and she said i would but he would have to be going out with me and i said i wasnt sure coz i know if i did go out with him he would just spend the whole time cheating on me and i dont want to but i feel presured please help i dont know what to do.