With regards to Marten and the future, I think its just more about having a sense of stability and knowing that you can have things covered. Yeah, Marten can be and is happy with his life as it is now, but it is still living from day to day and that isn't really sustainable in the long run. He's hitting stasis and stagnation while the people in his key relationships are changing some aspect of their lives. Hanners picked up sticks and took to travelling around the world, Faye and Bubbles not only began a relationship but also a business, Dora and Tai's relationship is growing and providing stability for the both of them.
With Claire taking her exam, she is getting to a stage where she needs to think about where her life is taking her and where she would like it to go. There is the possibility that if Claire does pass her exams, she might not be able to get a job locally. It might be time for Claire to spread her wings and leave, with Marten's arc potentially about growing or staying in stagnation.
Back to Dora and Tai, I agree with Emperor Norton, we've never seen indication that the two have ever had the talk. Now, this could be Tai joking or we might be seeing something shaking up, because Dora looks overwhelmed. If it is Tai joking, it could backfire big time. Whatever it might be, this storyline is setting the stage for something major.
Honestly, my original thought was that Claire failed her exam and having to deal with that. With Marten now, I thought it could be about him deciding whether or not he's going to "grow up". And now Tai and Dora. And with less than a month until the 4000th comic, we're building up to something big.