Lets just finish this.
You think that a child is a part of or the responsibility of the woman hosting it, correct?
Then its not your call. That is all there is to it. Just because you are in a "relationship" does not give you ownership of another person's body in any way shape or form.
If you do not believe that the child is part of or the responsibility of the woman, then you believe you have ownership because you helped create something. Unfortunately 99.9999999% of the work towards childbirth is the responsibility of the woman, again, so you really aren't a deciding factor in any of this.
Also, please stop being an idiot and thinking childbirth is something a person should have to suffer through for your selfish desires.