Just food for thought, what happens when sex becomes less exciting with your second partner & you become attached to her too? Do you look for a third? What if one of the first two want to start a family? They are not going to be thrilled being pregnant while you are out sleeping with others. One or the other may even come to resent you.
Or, what if your first partner really clicks with someone else , just as the great sex is slowing down with your second partner, & your draw to her (the 2nd) isn’t quite as strong? What if your 1st decides she wants a child with someone who isn’t you at some point? Or any of your partners down the road decide a child with you would be nice, & your 1st partner disagrees?
There are many more scenarios, but my point is that you may want to put some agreed, general ground rules in place before you get too far down the road. (They can always be revised down later.) You should both probably stop & evaluate your relationship together for a moment to figure out what, if anything, you want or expect from it in the future. By doing so, you both will be on the same page, & it will manage expectations.
Ultimately, however, I fear someone will want something the other doesn’t, someone’s feelings will get hurt, someone will feel left out, or jealousy will pop up, even with the best laid plans.
I wish you both the best & hope you can find happiness in life exactly as you would like. Cheers!