First time Sex

So I had sex with my boyfriend for the first time last night and I didn't even mean for it to go that far. We were doing foreplay in the pool like he was playing with my clit and fingering me and I was jerking him and it turned into him insterting his penis for the first time. We were both virgins and didn't use a condom. I've taken multiple pregnancy tests and all are negative plus I'm on birth control. But I'm not sure if I want to do it again. I was waiting for marriage and it just kind of happened. I don't want to be mean to him and say I don't want it again or anything but what would the nicest way to tell him be? Tell him that I don't want to have sex again