if your not in the mood for sex a guy may ask if he can atleast put the head in for a few seconds or for long enough to jack off. nine times out of ten, putting the head in leads to sex anyway so its a stupid line. also if i'm hanging out with somebody new and not yet comfortable enough to have sex with him he'll ask if he can just "put the head in"

(as in put the head of the penis into ass/vagina without inserting the whole penis)