The question pertained to undergrad, not med school.
The only responses you've gotten are positive, and, surprise, they are all from married people (who are still married)!
OF COURSE you are going to miss out on many things if you get married as an undergrad (unless you're already "older"). Marriage, no matter how great or free one feels in such a relationship, demands a level of compromise. Granted, many people force themselves to believe that it is not really a compromise by saying that they don't have an interest in (blank) activity/experience - but that is a coping tactic. People who get married at such a young age do so because they don't want to be alone and they have grown so much with the person that they decide to marry that they end up very much complementing eachother in many regards. This is where the feeling of "(blank) is the only one for me" comes from.
Every choice is a trade-off. Yet, dow well you can swindle yourself into the advantages that one choice hold over another is up to you.