Sex for first time

I have been dating my boyfriend for a year today and he is wanting to have sex and I do too. He is 16 and I'm 15. The only thing I'm scared of is I have said all my life that I only want 1 sex partner my whole life because I mean that's how I was raised and taught since I was little. He says he is ready and I'm the only girl I want and he wants to marry me later on in life and have kids but what if he breaks up with me later on? Then there goes my 1 partner! Any advice on what to do?