I love sex and am very bored by baseball, so the “first base,” “second base,” “third base” metaphor has always seemed a bit silly to me. Making out is much more fun than running around a baseball diamond, if you ask me. But sexual baseball metaphors seem like they're here to stay, so we might as well figure out what they actually mean.
According to Know Your Meme, baseball as a sexual metaphor dates back to the post-WWII era in the United States, an era that, as writer Windy Sombat put it, “set up precedents in dating that led to what many consider ‘normal’ dating today.” Initially, “first base” meant kissing, particularly French kissing or making out. However, as time has gone by, that definition has gotten looser. For example, the top-rated Urban Dictionary definition of “first base” is, “The first step in a sexual relationship, involving making out or French kissing,” but the top-rated definition for “sexual bases” adds, “any above-the-belt touching is included in this base.”
Columbia University’s sex education resource Go Ask Alice agrees that “first base” is “kissing, including open-mouth (or French) kissing,” but clarifies that different people may mean different things by the term: “These [definitions] can vary among different people, so don't be embarrassed to ask your friends exactly what they mean when they talk about the different bases — you may find that you aren't the only one who needs some clarification on both the bases and the definition of sex.”
Some activists have argued that the baseball metaphor frames sex as something to be “won” from an opposing team, which doesn't have great implications for consent. As sex education resource Scarleteen puts it, “The value placed on ‘getting to a base,’ reveals a deep flaw in our culture's view of sex. Sex isn’t a ‘thing’ one can ‘get’ from another person. It's something wonderful that everyone has within themselves, that can be enjoyed alone or with others.”
So maybe consider doing away with baseball metaphors entirely — tell your friend, "I just had the best makeout sesh," or ask your partner, "Can I kiss you?" That’s a lot sexier than baseball, anyway.