im 20, a girl and have been with my partner for nearly 3 years, my partner is 24 and just like your partner has had sex other people, yes 15/16 is the age teenagers get curious and experiment pretty normal cant help you on the sexual partners part of the question although my partner told me his 1st time was when he was 14 and he is a boy im guessing we are older than yous, so hes had more than 3/4 partners. The moment we actually went wrong is when we asked our partners that question I to know most girls hes had sex with and once I see them talking or even see one of them. Im in a crap mood, Although our relationship is great, have 2 kids, its just one of those things that will never go away but will ease in time, if you do love her you will eventually kinda get over it. Just remember she hasnt cheated on you and this happened all before she knew you or knew you would be apart of her life. You cant take back what she did its just one of those things you have to live with when you really love someone ie. i hate how my partner speeds, you just need to look at it as one of their "cons" because after all that is how yous guys started your relationship just a casual fling. Dont think about the past sexual relationships just think about how your her last and she from now on always be yours