family giving me a hard time about baby's last name

I have an older child with my last name (father isn't in his life) this baby's father and I were only together for a month, knew each other for 3 months, before I found out I was pregnant. We moved in together and are in a relationship. He's very active in everything regarding our baby and very good with my older child as well. Sadly, we are just very different people and we hit walls a lot on some very basic relationship topics such as homes, cars, finances etc. and those things normally would have been sorted out in a relationship before you have a child but it happens. He's a great guy but I don't see myself marrying him but I'm trying to work through my hormones and differences for our son that will arrive in max. 3 weeks. Regardless, I know our son will have two very loving and active parents in his life so that's why I decided to give him my boyfriend's last name. My family thinks he should have the same last name as my older son and myself. I feel like it's a big deal for a child, especially a son, to receive their father's surname assuming he's a good guy/father and all. So, my older son and I will have my last name and this baby will have his father's.  Seems pretty understandable to me but I'm constantly hearing crap about it and it's weird considering it's my mom's side of the family saying it and it's not even their last name; it's my biological father's who hasn't been in my life... I like my last name though lol. Ugh, am I being reasonable with giving my son his father's last name even though his brother and I will most likely never have the same last name as him?