Wow this arc is bad. The "forced marriage" trope is terrible under normal circumstances, but this situation is so hilariously contrived it almost feels like a parody.
First off, Kara and marriage-man aren't literally the last titans. There are presumably at least hundreds of titans left or their species is doomed to die from inbreeding's genetic problems. As such, while Kara is understandably under pressure to have children, its not like she's going to kill off her species if she doesn't.
Second, pregnancy and marriage are entirely different things. I realize they probably don't have IVF in the titan world, but they have access to the human world where such things are commonplace. And even if that's not an option for some reason, people have employed variations on the "turkey baster" method of pregnancy for hundreds of years. And even if that's inexplicably not an option, they could just have awkward sex once. I don't see why marriage is a requirement. In fact, wouldn't it be better for the Titan species if they didn't get married, since Kara and Vivi could raise the child and marriage-man could go find another mate? I guess that runs into the "off-worlders can't visit the titan world" issue, which brings us to...
Third - the titan world's rules are contrived and silly. When we went to/from Vivi's world in prior chapters, Vivi just drew a circle and they traveled to the other world. Kara was literally dying from poison and she had no problems travelling. Yet somehow children are too weak to handle the ride to Kara's world, and foreign people like Vivi can't make the trip at all? I realize the author can make whatever world-jumping rules she likes, but this is feels extremely convenient.
Fourth: I don't understand why Kara couldn't raise her children in the modern human world (with Vivi). Like, even if we roll with the concept that kids and old people can't "handle the burden of travelling to another world," Kara could still get pregnant on the modern human world and just not go back. The "reason" given is that it would be "throwing away our pride as titans." But like, Titans are being genocided in her world. Her mother was literally killed by this genocide. Why on earth would Kara choose to raise a child there when a way safer alternative exists? The notion that Kara would care more about her pride than her child's safety is wildly out of character.
Fifth, I hate how this storyline has treated Vivi. She was mistreated by her family for so long, and when she finally forms a real connection, it's broken almost immediately. And the worst part is that the manga seems to put the blame for it falling apart on Vivi. We don't have a scene where Kara realizes how dumb her decision was, or how she betrayed both Vivi and the promise she made to her foster mother. Instead we have a scene where Vivi inexplicably didn't realize Kara had romantic feelings for her, and a side character tells her to be more honest with her feelings. I hate it.
I've ranted for far too many paragraphs, so I'm going to stop. Maybe future chapters will clarify Kara's feelings, and acknowledge how shitty she was to Vivi. Maybe there will be more exposition to make the marriage concept feel less contrived. Maybe Vivi will make a tearful speech about love, and everything will work out just fine. But regardless of what upcoming chapters do, this arc has left a bad taste in my mouth, and makes me not want to read this author's works in the future.