Last names

Becca • Loving my babies.🥰
So me & my boyfriend have been dating for a year and 3 months but have known each other since 2014... I'm now 7 months pregnant!! 
I'm having a hard time dealing with my MOM!! 
I told her that my child is going to have HIS LAST NAME & she turned her nose up & says "why?" "Yall aren't married" 
I personally LOVE his last name & I plan on marrying this man one day!! 
I just think it would be pointless to give her my last name, if I'm going to marry him one day & have his last name! I don't want to have to change MINE & HER last name when we get married so I don't understand the problem???! 
Anyone else dealing with this????