I've read it and am still usnure of what happened.
Seems like the guy was able to hold back at the end, put the tip in and retracted before it was all, so he didn't break the deal with the girl, but then she left anyway.
It seems that she did because he is a M (masochis) and the pain of being separated will make him happy, or the scar of their relationship being interrupted at his highest moment will make him unable to have a better match with anyone else.
It make no sense to a normal person because it have a very toxic couple.
Or the girl died right after the kiss, but doesn't seems like that
this one-shot was made almost 30 years ago, we will probably never have a full story and can only guess the right answer.