I’m confused:(

I’m with my Bf and I really do love him but I just have some concerns that I wanna run by y’all so please help me out!

1). Is it normal that my mind casually fantasizes about someone else?

2).He tells me he wants to marry me and all that. Him and I are incredibly young and him and I understand we have a lot of growing up to do. But he constantly tells me he’s going to marry me and it’s starting to get on my nerves. How do I tell him to lay-off on the whole “ marrying me” thing without hurting his feelings?

3).I’m scared to lose my virginity to him even though I’m 21, is that normal?