How do i get rid of unwanted ual fantasies or, is it possible my boyfriend will accept or encourage this..?

I am an 19 yr old female in a very loving and commited year and a half relationship and something very strange is happening, i'm starting to fantasize about having other men sometime even my boyfriends friends join in on our , i'm trying to hide these thoughts but they consume me! they've gotten so bad i toss and turn at night tingling with excitment as my mind wanders, with my boyfriend sleeping soundly beside me, or sometime when he touches me i close my eyes and imagne him and other hands caressing me. i dont know what ot do i feel terrible like i've cheated! i know my boyfriend would not find these thoughts acceptable. nor do i, i have never been promiscuous or anything, actually my boyfriend has always commented on how my pure innocence is such a turn on to him, so i dont even know where these thoughts are coming from,do women just turn into sluts! i'm very worried that these fantasies will lead me to say something to my boyfriend about them and he will leave me, or think i am a slut i've never kept anysecrets from him before and this is really driving me crazy what should i do?!