[All of this is beyond Chekov. He has only had a handful of prior relationships and, of those, only one has had a sexual element. It has been so different for him than for Kitty. Each relationship had ended amicably and without drama when either he or she needed to leave; there was no overlap, no break up, no hard feelings, no broken hearts, and no mess. Sadness, yes, and he still misses Salla zh'Tran, Tess, and Lucy--of course he does, he did love them--but there's no heartache. He had never been naive enough to think that any of those loves were forever and he knew that he would always return to space and his work, alone.
His relationships have been orderly where Kitty's have been messy, clean in a way that sounds decidedly unromantic because everyone involved knew that it was, by necessity, temporary. Chekov still loved them wholeheartedly and without reservation (he doesn't know how to love any other way), but his hopes and future and happiness were never tied to another person. They can't be. He moves too quickly and too often (and is too aware of that fact) to grow the kind of roots that, once severed, never stop hurting.
And Kitty's experiences have been incredibly dissimilar. He can't imagine being in a position where a number of people are involved and feelings are getting hurt and there's jealousy and multiple love stories being told at once, with each at a different point in the narrative. Before yesterday, he hadn't even thought about being involved in a relationship that had no set end date. He doesn't really understand her experience of love.
Again, it's a lot. Not just what Kitty's telling him now, but also what happened yesterday. He's beginning to understand that he could be getting into something far larger and scarier than he had thought.
He's here now, though. Kitty has his heart and it's too late to ask for it back.]
Thank you. [It's a brief kiss, but he's here for it.]
I would like to make this Kitty feel the same way someday. [He hugs her back tightly. Someday, which implies them being together for an indefinite amount of time, which is terrifying now that he's thinking about it. This is different and new and it's not going to be as clean as the other times he has been in love, but he means it.]