You know, this makes me wonder: If you suck on your girlfriend's boobs long enough, will she eventually start giving milk? I know a girl needs to be pregnant to begin producing lactation milk, but would her body start just from constantly sucking them? Hmmm....
Milk production is based on the amount of the hormone prolactin that is in the system. Normally prolactin production is stimulated by the pregnancy process, but it can be produced by other means- Some medications have the side effect of raising prolactin levels, and there are ways to trick the body into producing milk/colostrum for both females *and* males.
The main problem is there’s usually no point in stimulating milk production outside of having a baby to feed; it’s very inconvenient for both the person lactating and their sig other to have to deal with. Sexy, sure... but the wet bedsheets and ruined clothes aren’t worth.