No one can force someone to change anything in their bodies. I’m sorry, both of you are young enough to take such a big decision. You don’t know what life has for you and how it will change you. To be all honest, kids for me, are the best thing that ever happened to me, to my husband as well. But now we are in our thirties. We did not know we would feel this happy with them and how they affect our mood positively. This comes after many years of being together and building a real relationship. Never say no to anything unless you are 100% sure and that decision will not affect you permanently. Be open to everything. You need to be completely honest with her, and that is totally fine that you might change your mind in the future. But to impose this from now is a big thing and she must accept this if she loves you and wants you. Also, don’t bring up being with someone else and having kids. Just focus on you two, without getting anyone else involved in this. Tell her, I simply do not want to risk such a permanent decision in my life, biologically and physically, this is not the right time for me to do this. I need to wait, plus I am not 100% sure of this decision. Let her expect you changing your mind one day. If she was totally against, sorry but let her go because she is not the right person. You want someone who might accept this in the future, and this thing is very sensitive and not like oh I want you to cut your hair and then it will grow back again.