Hello Brenda

This is an interesting poem because my wife and I were discussing this a few days ago. We were quite lucky because we met when we were teenagers and so the insecurities of past relationships never plagued us too much (her a little more as I am a bit older) and I know that some people do get very jealous - because of the lack of confidence they have in themselves - when thinking about their partners with other people.
As you so rightly say, you cannot wish to claim anyone's past that didn't involve you, but as you also point out, if they are with you now, then you can claim the much more important future.

All the very best Brenda,
Ben