My reasoning against premarriage sex is mostly about religion. but I think the most important thing in this regard is that you really think about such choices and only move forward in the bedroom if you really love someone for a longer period of time. This way it is really precious what you give. Side effects of this is that your risk on getting an std is far lower and you probably won't be surprised with an unwanted pregnancy.
My reasoning against masturbation is mainly so you can set the priority on the other person instead of yourself.