By one guy to another guy = bonding, joke about taboo subject, communicate relationship position to outsider. I suppose the ancient phrase "pitching woo" MIGHT have a baseball source, but it may be too old.
Foreplay to measure the boundaries and relative emotional to physical position of the people in regards to the relationship? Rounders has six to eight bases, so we Americans would become too impatient waiting through SEVEN steps to get to Home Run land).
Second Base: Rounding second involves copping a feel. If it were up to me, I'd include the whole feeling up business in first base.
Second base would involve touching down there and oral would be placed at third. It has been suggested that some of the confusion might be cleared up by designating "minor" and "major" leagues of play.
And there's the whole scandal about steroids, making one's 'package' shrink in fear. In the gay male parlance, I understand there's talk about pitchers and catchers, which conversations are not at all my cup of tea (I spent some of my early formative years in English schools in London, so I drink tea, and think Rounders before I think baseball, to this day.