Pretty much what Tonberry said
Do you really have to think so much at this point in time? Really, is there a goal or time frame for your boyfriend to get married and have kids?
If not, why not just enjoy what you have now with the knowledge that things will likely change. If he is truly monogamous and wants what most other monos do ....the spouse and children being the most common, then eventually he may have to stop seeing you to become appealing to a mono woman. That is the sad thing of being mono; we say good bye to one love to embrace another. Sad but true...love is not a choice.
Good luck and just enjoy each other for now