Do you have the original Gunslinger or the revised edition? King re-updated the book while he was writing the end of the series. To make it match up more.
The sex wasn’t for thwarting - the spider was already thwarted at that point - but for bonding.
I’ll second and third the general concensus that it’s not necessay to have read any other works. However, having read everything that he’s written it’s facinating how he can tie them together. Kind of like a god that sees all in his own universe.