This isn't the ending, the next chapter will be the final one, I'm not sure why people think this was the last one. I think this is actually a good set up for the conclusion as the main conflict of the story was pretty much resolved in this chapter. Not sure who Futaba is marrying, could be a random guy or some side character BUT Masumi's feelings for Futaba were never resolved ( I don't think?) so maybe her? Thats just speculation though. This manga was ultimately about the individual character's self-discovery and acceptance of themselves, and they all showed their growth in this chapter (i.e. screaming I LOVE YOUUU at the beach). I personally didn't think Futaba and Taichi would stay together forever as, realistically, most highschool relationships just don't work out. Also, I think one of the characters getting married is an apt ending for a story about adolescence.
I apologize for the rant.