People usually decide to have kids with just one of the partners, usually the spouse if they are married or the person they are living with. It is often tempting to have kids with the NRE-partner, but if this partner is the one you have known for the least amount of time or the one you are less entangled with (which is often the case), keep in mind the legal, financial and practical ramifications, including that having kids is a long term investment and the persons you choose to be the kid's social and/or bio dad(s) should be committed to this function also if/when their relationship(s) with you fall apart.

If you want kids with a non-spouse/ non-cohabitating partner or with both of or your partners (as opposed to one), consider if living separately would facilitate the kind of family you want to create.