There are so many variables to this question that it makes it difficult to imagine "the" answer!
The OP is in a new relationship (impressing a new partner probably means carrying more gear). He is also the more experienced backpacker (the more experienced person carries more gear). He's also probably physically larger/stronger (the larger/stronger person carries more gear).
So much will depend on each person's comfort level, expectations, and the couple's "dynamic".
My husband is a few inches taller, and definitely stronger than I am. He's also a lot like a sled dog — he's excited, he wants to GO! Giving him more than his fair share of the weight keeps his energy levels more reasonable for normal humans. We hiked the JMT at a 2-3 mph pace, with me in front (I set a good, steady pace), and that worked really well for us both.
I remember passing a couple along Woods Creek: he was blithely skipping along WAAAAY out in front, and she was struggling, sweating, and muttering obscenities at him (he was too far ahead to hear). I wondered what the atmosphere at their campsite that night would be like…hmmmmmmm, not good, methinks! Don't be THAT GUY. Well, unless you're trying to get rid of her… ;^)