Things seem to have veered off little from the three specific questions you asked.
Here's how I interpreted what you wrote. You had plans to get married he was excited and enthusiastic about the idea of marrying you. You pushed this new lifestyle ... he took to it quickly. His thoughts on marriage shifted from the initial conversation and then certainly in regards to his feeling for his new partner. And also maybe the logic of marriage got lost there . His new single partner is talking about having a child, parenthood. You know all to well the complexities of that world. The speed and the amount of change that's occurred in such a short space of time is disconcerting. It seems very natural to pick apart the negative trend lines and extrapolate future events, especially around the topic having a baby. And the regret of having caused this on yourself seems very understandable.
On one hand the relationships are separate and independent of one another. On the other hand everything is connected and interwoven ,ala "its a wonderful life ", compersion, the expansion of love.
The question is to have this new lifestyle what are you willing to give up, or invest. Whats the risk of loss. How much loss can you tolerate. A percentage or total. If you played with scenarios could you be secondaries to each other (if you believe in a hierarchical model).