Home base (right after you hit the ball): Holding hands, kissing, making out, touching, "heavy petting" and "grinding" (haha, I hate those terms so much), sex by hand, coitus, saying hello, friending them on facebook, spying on them through their window, marrying them when a dowry is involved, etc. In other words, none of these mean anything.
First base: Being able to actually enjoy spending time with the person and have a conversation that you don't have to construct for them.
Second base: Being able to spend time with the person and feel like they have a positive influence on you.
Third Base: Being able to spend time with the person and feel comfortable. You can let your guard down and say what comes to mind without filtering, knowing that even if you say something silly they know you enough to know that you are a good person overall. Being able to relax and be silent with a person while enjoying together the simple things of life, like a nice car ride or a good song.
Home run: Feeling like the person understands what you are thinking. Having an equal emotional power balance; not feeling like you are older than them, regardless of their age. Not feeling like you understand them more than they understand themselves, and even, heaven forbid, they can provide insight about parts of yourself that you previously were confused. Being able to have long flowing conversations about your emotions and thoughts about people and the universe. Knowing that this sort of conversation is way more effective than any psychotherapy or medication.
Physical contact is such a red herring (out in left field, so to speak). I think of it like alcohol. Drinking alone or with strangers is only good for passing the time. But drinking with friends is one of those simple good things in life to enjoy. Sex with someone you don't feel close to is overrated. Occassionally I have the desire, but in the cases that I follow through I end up having a bad emotional hangover (lonliness). Sex with someone who can complete your sentences is an emotional ******.
My theory is that this heirarchy of physical contact milestones (expressed via baseball) is just a way to organize our levels of guilt. It's all silly though because in reality love has nothing to do with any of it. But I'm not knocking the bases analogy though. It's a useful tool for processing exactly what physical contact means in a relationship. I just wish society could move past this assumption that sex=guilty because it's keeping us from putting sex in it's proper place.
Hint: If you are thinking of marrying someone who is "eh, alright", with the assumption in the back of your mind that when you marry and can have sex that love will find a way, you should definitely reconsider. Ignore your youth pastor. Sex (or saving your virginity) isn't a marriage saver or marriage breaker. The strength of a marriage is determined by the relationship itself. I traded my virginity for this lesson and I've never regretted it.