I don't think they had THE SEX but they probably got a little hot and bothered. Probably a good makeout session, maybe some sexy carressing, groping, jacking off, sucking, etc (last few seem unbelievable). That boy Kouichi doesn't seem like the guy who would go off plan, especially on the whole baby thing. I don't think he prepared condoms too (and Sachi probably doesn't take birth control).
BUT YOU BET YOUR ASS THAT THAT BOY IS BUYING VALUE PACK TROJAN FOR NEXT TIME!
P.S. I don't buy "pull out = perfectly fine" like puleezzz...that boy is 100% virgin he won't know what hit him till it's out (or in??).