the sex also shows the personality of the characters to. they arent the goody goody characters that would rather read a textbook than look at a girl (or guy depending on which char...) so it tells you that at least they are more carefree and not afraid to have sex spontaniously.
imagine if kei became shy and stumbled on words when he first saw the naked girl (forget names -_- so long ago... the suicide girl.. in the first meeting) it would ruin the story. Of course you could probably go with out all the sex but perhaps the author is using it as a way of stress relief with all the killing and people dieing.
I agree wif your stress relief point...missions are so bloody...well...bloody...