If your SO doesn't care then it sounds like it's totally up to you. Have you decided on the first name? Knowing what the last name is going to be will help you to pick out a first name and vice versa. 

I would try on both names for a couple days. In your head just imagine that you have decided that the baby is going to have your name or your husband's last name. Write it down, imagine yourself telling other people about her name, imagine her going to school etc... Then do the same thing the next day or two with the other name.  I truly believe with decisions like this you need to take your time and sleep on them. Your brain continues to try to make a decision while you're sleeping! 

I think it's normal to want to have the same last name as your baby whether you are married or not. 

If your partner is okay with the baby having your last name, that is what I would do. Like you said, you can change it if you decide to marry him and take his name.

I would NOT hyphenate it. I have a friend who did that and she regretted it, as it became a nightmare.

Last names are cultural. In many parts of the US, it is mainstream for a child to have the dad's last name whether the parents are married or not. 

Marrying him or not marrying him, the child is still his. You live with him and plan to keep doing so. Dad's name will be on the birth certificate. Why plan to go to probate court in the future to change it? Why plan to pay the probate court fee? Why not just do it now and get it over with?

If I'd do anything, I'd not change my name when I married. Changing your name as an adult woman can be a hassle. 

My DH and I are married, but I didn't change my name, so we have different last names. We have a daughter and she has DH's last name. I considering pushing to give her my last name, but ultimately decided against it. In all honesty, I didn't want BM, SSs, or ILs to try to use the last name against my child to try to imply she was less worthy of being part of the family. All are already all jumpy about DH giving too much attention to our daughter and "ignoring" or "replacing" SSs, which he is not doing.

I decided against hyphenating because I have friends who have hyphenated and it's been a pain for them. 

I also have friends whose mothers kept their names, so they grew up with different last names than their mothers and everything was fine. I have to explain that my child has a different last name, but I've had to explain that about my husband and I since we got married. 

I would give the baby my last name.  If I ever married the father and changed my name, it would be easy enough to change the child's name.  

DH and I are married but I didn't change my last name.  All of our legal documents have both of our different names on them.  The only thing I did do was hypenate on FB and personal emails to make it "official"  LOL  The SS's friends do call me Mrs. DH.

I do not have children of my own, but when I was younger I always said that if I had a boy, I would give him my last name.  I am the last surviving member of my surname.  It would have been nice to carry it on.

Honestly it's a very cultural thing. While in my area most kids have their fathers name in some places it's customary for the baby to take the mothers last name or to have both. To me that means there's no one right way to do things, so go with whatever you want. If you two don't have strong preferences then just do what makes you and dad happy and leaves both of you feeling satisfied with the situation. Make your choice with love and care for the child and your partner. Anyone else's opinion doesn't really matter. Hugs and congratulations 

Lets say your last name is Smith and SO's last name is Jones.

Baby's first name is Lilly

You could name her Lilly Jones Smith OR Lilly Smith Jones???

It's kinda cool to intertwine both parents name into the babys name.

Personally I believe kids should have the biological fathers last name.

I didn't change my name when I married DH and my BS has my last name. When we had BD I incorporated my last name as one of BD's middle names. My DH who was very old fashioned felt women should take a man's name once they marry. I pointed out to him, he has two daughters...and so does his brother...therefore his family name will die out given the old crusty tradition. I pointed out my name will go on because of my BS and BD having my last name as a middle name. After that DH changed his mind and likes the idea of women keeping their name.

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I would go with both last names and hyphenate. You could later hyphenate if you do get married as well. You will always be a part of your family and wanting your child to have that name too shouldn't be hard for your SO to understand. 

My SIL is from Columbia. There, the child has 2 last names, Father's and then Mother's Maiden. 

Hyphenate.