Why does my boyfriend wana get me pregnant so bad??!?

Ive known him for 5 years but been with him for almost a year. Ive made it 100% clear that marriage is 1st then children. I am in no rush for marriage or children i actually appreciate it being jus me n him. Besides i come frm a family of 7[single parent!] his is 4 w/ both parents. I love him to tears and i want 2-3 kids but i am in NO rush. Im only 19 n he's 20! He loves me a lot and wants to get married n a few years and then start on kids...he even comes up with names! [ok i participate too] but he's fixated on marrying me and gettin me knocked up! Whats his deal?! Bcuz im not mentally or emotionally ready for kids and he totally is. I dont want this to be the fall of out relationship!