me and my girlfriend SeXStoRY

Iv been with my girlfriend for two years and a bit now so far things have been good real good i love the sex its great but shes the only girl iv ever slept with and i keep thinking am i missing out? i love this girl and its not only the sex that is good she is smart funny loving but i do feel abit weird knowing shes the only one iv done it with shes had sex with one other boy (well she says lol) but i belive her i dont wana cheat on her cos i know she'll leave me but i gota see wat its like with another girl please give me some advice someone!